feat(scroll): let users pick a crisp speed for their own panel

Whole-pixel motion was previously only available at multiples of the refresh
rate -- 100, 200, 300 px/s on a 100Hz panel. 100 px/s crosses a 256px panel in
2.6s, which is brisk for reading, and everything slower had to blend (blur) or
repeat frames unevenly (judder). There was no way to ask for 50 px/s and get
clean motion.

SwapOnVSync takes a framerate_fraction the display manager never passed. It
holds each frame for N panel refreshes; the panel keeps refreshing at its full
rate throughout, so holding costs nothing in flicker and only changes how often
a NEW image is presented. That turns 50 px/s into one whole pixel every second
refresh instead of half a pixel every refresh.

The crisp speeds are therefore refresh_hz / hold * pixels_per_frame, and that
ladder depends on the panel: a Pi Zero on a long chain has a different set of
good speeds from a Pi 4 on a short one. crisp_ladder() enumerates them and
solve_crisp() picks the best match for a requested speed.

solve_crisp weights motion quality rather than picking the numerically nearest
entry, which matters more than it sounds. Asked for 30 px/s, nearest-by-value
answers 28.6 -- 2px jumps at 14fps -- over 33.3, which is single-pixel motion
at 33fps and obviously better on the panel. The target is also clamped into the
ladder's range first, because relative error saturates near 1.0 for a target
far outside it and the quality penalty would otherwise answer "10000 px/s" with
the slowest entry.

configure() snaps to the ladder and applies the hold when given a display
manager. Without one the hold silently cannot happen and motion falls back to
fractional pixels, so it warns rather than failing quietly. set_frame_hold()
resets to 1 when scrolling stops, so one plugin's pacing cannot leak into
whatever is on screen next.

scripts/scroll_speeds.py is the user-facing part: it prints the ladder for the
configured rate, measures what the panel ACTUALLY manages (--measure, for
hardware that cannot reach its configured limit), highlights the nearest option
to a wanted speed, and demos one live. It never starts or stops the display
service itself -- doing that inside a script stranded the panel twice today.

Speeds below ~20 px/s remain stepped regardless. That is the pixel pitch, not a
software limit.

Also fixes the dirty-tracking test spy, which stubbed SwapOnVSync with a
single-argument function and would have masked the new call as a failed push,
and rewrites a configure() test that had started passing for the wrong reason:
it asserted a judder warning, which snapping now prevents, and was matching the
unrelated "hold could not be applied" warning instead.

183 tests pass on the Pi.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

scripts/scroll_speeds.py

@@ -0,0 +1,238 @@
+#!/usr/bin/env python3
+"""Show and try the scroll speeds your panel can display cleanly.
+
+Motion looks smooth when the strip advances a WHOLE number of pixels per panel
+refresh. Anything else has to blend two columns (which on pixel-font text reads
+as shimmer) or repeat frames unevenly (which reads as judder). So the speeds
+worth using are not arbitrary -- they are
+
+    refresh_hz / frame_hold * pixels_per_frame
+
+for whole numbers of frame_hold and pixels_per_frame, and that ladder depends
+on how fast YOUR panel actually refreshes. A Pi Zero driving a big chain will
+have a completely different set of good speeds from a Pi 4 driving a small one.
+
+    # what can this panel do? (no hardware needed, uses your configured rate)
+    python3 scripts/scroll_speeds.py
+
+    # measure what the panel ACTUALLY manages, rather than what is configured
+    sudo systemctl stop ledmatrix
+    sudo python3 scripts/scroll_speeds.py --measure
+    sudo systemctl start ledmatrix
+
+    # what would a 60Hz panel offer?
+    python3 scripts/scroll_speeds.py --hz 60
+
+    # try one on the panel
+    sudo systemctl stop ledmatrix
+    sudo python3 scripts/scroll_speeds.py --demo 50
+    sudo systemctl start ledmatrix
+
+This script never starts or stops the display service itself -- that is left to
+you, so a crash here can never leave the panel dark.
+"""
+from __future__ import annotations
+
+import argparse
+import json
+import os
+import sys
+import time
+from pathlib import Path
+
+sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
+
+from src.common import scroll_config  # noqa: E402
+
+CONFIG = Path(__file__).resolve().parent.parent / "config" / "config.json"
+
+
+def load_hardware():
+    try:
+        with open(CONFIG, encoding="utf-8") as handle:
+            return (json.load(handle).get("display") or {}).get("hardware") or {}
+    except (OSError, ValueError):
+        return {}
+
+
+def build_options(hardware, refresh_override=None):
+    from rgbmatrix import RGBMatrixOptions
+
+    o = RGBMatrixOptions()
+    o.rows = int(hardware.get("rows", 32))
+    o.cols = int(hardware.get("cols", 64))
+    o.chain_length = int(hardware.get("chain_length", 1))
+    o.parallel = int(hardware.get("parallel", 1))
+    o.brightness = int(hardware.get("brightness", 80))
+    o.hardware_mapping = hardware.get("hardware_mapping", "regular")
+    o.pwm_bits = int(hardware.get("pwm_bits", 11))
+    o.pwm_dither_bits = int(hardware.get("pwm_dither_bits", 0))
+    o.pwm_lsb_nanoseconds = int(hardware.get("pwm_lsb_nanoseconds", 130))
+    o.led_rgb_sequence = hardware.get("led_rgb_sequence", "RGB")
+    o.scan_mode = int(hardware.get("scan_mode", 0))
+    o.row_address_type = int(hardware.get("row_address_type", 0))
+    o.multiplexing = int(hardware.get("multiplexing", 0))
+    o.gpio_slowdown = int(hardware.get("gpio_slowdown", 2))
+    o.limit_refresh_rate_hz = (
+        int(refresh_override) if refresh_override is not None
+        else int(hardware.get("limit_refresh_rate_hz", 0))
+    )
+    return o
+
+
+def open_matrix(hardware, refresh_override=None):
+    """Construct the matrix, or explain why it will not open."""
+    if os.geteuid() != 0:
+        sys.exit("this needs root for GPIO access - rerun with sudo")
+    try:
+        from rgbmatrix import RGBMatrix
+    except ImportError:
+        sys.exit("rgbmatrix is not installed on this machine")
+    try:
+        return RGBMatrix(options=build_options(hardware, refresh_override))
+    except Exception as exc:  # pragma: no cover - hardware dependent
+        sys.exit(
+            "could not open the panel ({}).\n"
+            "If the display service is running it owns the GPIO - stop it first:\n"
+            "    sudo systemctl stop ledmatrix".format(exc)
+        )
+
+
+def measure_refresh(hardware, seconds=6.0):
+    """Actual refresh rate, by running uncapped and timing the swaps.
+
+    SwapOnVSync blocks until the panel's next refresh, so an unthrottled loop
+    runs at exactly the panel's rate. This is what an older Pi or a longer
+    chain will really give you, as opposed to whatever limit_refresh_rate_hz
+    optimistically asks for.
+    """
+    matrix = open_matrix(hardware, refresh_override=0)
+    canvas = matrix.CreateFrameCanvas()
+    canvas = matrix.SwapOnVSync(canvas)  # discard the first, it includes setup
+    frames = 0
+    started = time.perf_counter()
+    while time.perf_counter() - started < seconds:
+        canvas = matrix.SwapOnVSync(canvas)
+        frames += 1
+    measured = frames / (time.perf_counter() - started)
+    matrix.Clear()
+    return measured
+
+
+def demo(hardware, target, seconds):
+    """Scroll text at the crisp speed nearest `target`."""
+    from PIL import Image, ImageDraw, ImageFont
+
+    hz = float(hardware.get("limit_refresh_rate_hz") or scroll_config.DEFAULT_REFRESH_HZ)
+    choice = scroll_config.solve_crisp(target, hz)
+    print("asked for {:.0f} px/s -> {}".format(target, choice.describe()))
+
+    matrix = open_matrix(hardware)
+    canvas = matrix.CreateFrameCanvas()
+    W, H = canvas.width, canvas.height
+
+    font = None
+    for path, size in (
+        (str(Path(__file__).resolve().parent.parent / "assets/fonts/PressStart2P-Regular.ttf"), 16),
+        ("/usr/share/fonts/truetype/dejavu/DejaVuSansMono-Bold.ttf", 26),
+    ):
+        try:
+            font = ImageFont.truetype(path, size)
+            break
+        except OSError:
+            continue
+    if font is None:
+        font = ImageFont.load_default()
+
+    text = "  {:.0f} px/s  ***  THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG  ***".format(
+        choice.pixels_per_second)
+    box = ImageDraw.Draw(Image.new("RGB", (8, 8))).textbbox((0, 0), text, font=font)
+    tw, th = box[2] - box[0], box[3] - box[1]
+    reps = max(2, (W * 3) // max(tw, 1) + 1)
+    strip = Image.new("RGB", (tw * reps, H), (0, 0, 0))
+    draw = ImageDraw.Draw(strip)
+    for i in range(reps):
+        draw.text((i * tw, (H - th) // 2 - box[1]), text, font=font, fill=(255, 210, 60))
+
+    offset = 0
+    frames = 0
+    started = time.time()
+    while time.time() - started < seconds:
+        window = strip.crop((offset, 0, offset + W, H))
+        if window.width < W:
+            whole = Image.new("RGB", (W, H), (0, 0, 0))
+            head = strip.crop((offset, 0, strip.width, H))
+            whole.paste(head, (0, 0))
+            whole.paste(strip.crop((0, 0, W - head.width, H)), (head.width, 0))
+            window = whole
+        canvas.SetImage(window)
+        canvas = matrix.SwapOnVSync(canvas, choice.frame_hold)
+        offset = (offset + choice.pixels_per_frame) % strip.width
+        frames += 1
+    elapsed = time.time() - started
+    print("  {} frames in {:.1f}s = {:.1f} fps = {:.1f} px/s actual".format(
+        frames, elapsed, frames / elapsed, frames * choice.pixels_per_frame / elapsed))
+    matrix.Clear()
+
+
+def print_ladder(hz, highlight=None):
+    print("")
+    print("Whole-pixel scroll speeds at {:.1f}Hz refresh".format(hz))
+    print("(the panel refreshes at {:.0f}Hz for every one of these - holding a "
+          "frame costs no flicker)".format(hz))
+    print("")
+    for entry in scroll_config.crisp_ladder(hz):
+        if entry.pixels_per_second > hz * 3:
+            break
+        mark = " <-- nearest to {:.0f}".format(highlight) if (
+            highlight is not None
+            and entry.pixels_per_second == scroll_config.solve_crisp(highlight, hz).pixels_per_second
+        ) else ""
+        print("  " + entry.describe() + mark)
+    print("")
+    print("Set one in config.json as pixels per second, e.g.")
+    print('  "display_options": {{"scroll_pixels_per_second": {:.0f}}}'.format(
+        scroll_config.solve_crisp(highlight if highlight else hz / 2, hz).pixels_per_second))
+
+
+def main():
+    ap = argparse.ArgumentParser(description=__doc__,
+                                 formatter_class=argparse.RawDescriptionHelpFormatter)
+    ap.add_argument("--hz", type=float,
+                    help="refresh rate to compute the ladder for (default: your config)")
+    ap.add_argument("--measure", action="store_true",
+                    help="measure the panel's real refresh rate (needs root, service stopped)")
+    ap.add_argument("--demo", type=float, metavar="PXPS",
+                    help="scroll text at the crisp speed nearest this (needs root)")
+    ap.add_argument("--seconds", type=float, default=15.0, help="demo duration")
+    ap.add_argument("--want", type=float, metavar="PXPS",
+                    help="highlight the entry nearest this speed")
+    args = ap.parse_args()
+
+    hardware = load_hardware()
+    configured = float(hardware.get("limit_refresh_rate_hz") or 0)
+
+    if args.demo is not None:
+        demo(hardware, args.demo, args.seconds)
+        return
+
+    if args.measure:
+        measured = measure_refresh(hardware)
+        print("measured panel refresh: {:.1f}Hz".format(measured))
+        if configured:
+            print("configured limit_refresh_rate_hz: {:.0f}".format(configured))
+            if measured < configured * 0.95:
+                print("  -> the panel cannot reach the configured rate; the ladder")
+                print("     below uses what it actually manages")
+        print_ladder(measured, args.want)
+        return
+
+    hz = args.hz or configured or scroll_config.DEFAULT_REFRESH_HZ
+    if not args.hz and not configured:
+        print("no limit_refresh_rate_hz in config; assuming {:.0f}Hz".format(hz))
+        print("run with --measure to find your panel's real rate")
+    print_ladder(hz, args.want)
+
+
+if __name__ == "__main__":
+    main()