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>

test/test_scroll_config.py

@@ -10,6 +10,9 @@ sys.path.insert(0, str(Path(__file__).resolve().parent.parent))
 
 from src.common.scroll_config import (  # noqa: E402
     DEFAULT_PIXELS_PER_SECOND,
+    MAX_PIXELS_PER_FRAME,
+    crisp_ladder,
+    solve_crisp,
     MAX_PIXELS_PER_SECOND,
     MIN_PIXELS_PER_SECOND,
     ScrollSettings,
@@ -40,6 +43,16 @@ class FakeHelper:
         self.target_fps = fps
 
 
+class FakeDisplayManager:
+    """Records the frame hold configure() applies."""
+
+    def __init__(self):
+        self.hold = None
+
+    def set_frame_hold(self, refreshes):
+        self.hold = refreshes
+
+
 class MinimalHelper:
     """An older helper exposing only set_scroll_speed."""
 
@@ -189,11 +202,60 @@ class TestConfigure:
         configure(helper, {"display_options": {"scroll_pixels_per_second": 100.0}})
         assert helper.speed == 100.0
 
-    def test_logs_the_warning_when_speed_will_judder(self, caplog):
-        helper = FakeHelper()
+    def test_snapping_removes_the_judder_warning(self, caplog):
+        """50px/s cannot be shown in whole pixels at 100Hz *per refresh*, but it
+        can as 1px every 2nd refresh -- so once snapped there is nothing to warn
+        about. resolve() alone still warns; configure() resolves it."""
+        assert "judder" in resolve(
+            {"display_options": {"scroll_pixels_per_second": 50.0}},
+            refresh_hz=100).warning
+        helper, dm = FakeHelper(), FakeDisplayManager()
         with caplog.at_level(logging.WARNING):
-            configure(helper, {"display_options": {"scroll_pixels_per_second": 50.0}})
-        assert any("judder" in r.getMessage() for r in caplog.records)
+            settings = configure(
+                helper, {"display_options": {"scroll_pixels_per_second": 50.0}},
+                display_manager=dm)
+        assert settings.warning is None
+        assert not [r for r in caplog.records if "judder" in r.getMessage()]
+
+    def test_warns_when_a_hold_is_needed_but_cannot_be_applied(self, caplog):
+        """Without a display manager the hold silently does not happen, and the
+        motion falls back to fractional pixels. That must not pass quietly."""
+        with caplog.at_level(logging.WARNING):
+            configure(FakeHelper(), {"display_options": {"scroll_pixels_per_second": 50.0}})
+        assert any("frame hold" in r.getMessage() for r in caplog.records)
+
+    def test_applies_the_frame_hold_to_the_display_manager(self):
+        dm = FakeDisplayManager()
+        configure(FakeHelper(), {"display_options": {"scroll_pixels_per_second": 25.0}},
+                  display_manager=dm)
+        assert dm.hold == 4, "25px/s at 100Hz is 1px every 4th refresh"
+
+    def test_full_speed_needs_no_hold(self):
+        dm = FakeDisplayManager()
+        configure(FakeHelper(), {"display_options": {"scroll_pixels_per_second": 100.0}},
+                  display_manager=dm)
+        assert dm.hold == 1
+
+    def test_snaps_to_the_nearest_crisp_speed_and_reports_both(self):
+        s = configure(FakeHelper(), {"display_options": {"scroll_pixels_per_second": 45.0}},
+                      display_manager=FakeDisplayManager())
+        assert s.requested_pixels_per_second == 45.0
+        assert s.pixels_per_second == 50.0
+        assert s.crisp is not None and s.crisp.pixels_per_frame == 1
+
+    def test_snapping_can_be_turned_off(self):
+        s = configure(FakeHelper(), {"display_options": {"scroll_pixels_per_second": 45.0}},
+                      snap_to_crisp=False)
+        assert s.pixels_per_second == 45.0
+        assert s.crisp is None
+
+    def test_helper_target_fps_matches_the_presentation_rate(self):
+        """At a hold of 4 the panel still refreshes at 100Hz, but frames are
+        presented at 25/s -- that is the rate the helper should pace to."""
+        helper = FakeHelper()
+        configure(helper, {"display_options": {"scroll_pixels_per_second": 25.0}},
+                  display_manager=FakeDisplayManager())
+        assert helper.target_fps == pytest.approx(25.0)
 
     def test_returns_settings_describing_the_source(self):
         s = configure(FakeHelper(), {"display_options": {"scroll_speed": 2.0, "scroll_delay": 0.01}})
@@ -211,3 +273,80 @@ class TestRefreshFromConfig:
                                      "nonsense", {"display": {"hardware": {"limit_refresh_rate_hz": None}}}])
     def test_falls_back_to_the_default(self, cfg):
         assert refresh_hz_from_config(cfg) == 100.0
+
+
+class TestCrispLadder:
+    """Whole-pixel speeds available on a given panel."""
+
+    def test_every_entry_is_exactly_reachable(self):
+        for c in crisp_ladder(100):
+            assert c.pixels_per_second == pytest.approx(
+                c.refresh_hz / c.frame_hold * c.pixels_per_frame)
+
+    def test_sorted_slowest_first(self):
+        speeds = [c.pixels_per_second for c in crisp_ladder(100)]
+        assert speeds == sorted(speeds)
+
+    def test_no_duplicate_speeds(self):
+        speeds = [round(c.pixels_per_second, 3) for c in crisp_ladder(100)]
+        assert len(speeds) == len(set(speeds))
+
+    def test_duplicates_resolve_to_the_smaller_step(self):
+        """100px/s is 1px every refresh or 2px every 2nd; prefer the former."""
+        entry = next(c for c in crisp_ladder(100)
+                     if c.pixels_per_second == pytest.approx(100.0))
+        assert entry.pixels_per_frame == 1
+        assert entry.frame_hold == 1
+
+    def test_ladder_scales_with_the_panel(self):
+        assert any(c.pixels_per_second == pytest.approx(60.0)
+                   for c in crisp_ladder(60))
+        assert any(c.pixels_per_second == pytest.approx(30.0)
+                   for c in crisp_ladder(60))
+
+    def test_full_refresh_speed_is_present(self):
+        for hz in (60, 75, 100, 120):
+            assert any(c.pixels_per_second == pytest.approx(float(hz))
+                       for c in crisp_ladder(hz))
+
+
+class TestSolveCrisp:
+    def test_exact_targets_are_matched_exactly(self):
+        for target in (100, 50, 25, 20):
+            assert solve_crisp(target, 100).pixels_per_second == pytest.approx(target)
+
+    def test_prefers_smooth_motion_over_raw_proximity(self):
+        """30 -> 33.3 (1px, 33fps), not 28.6 (2px at 14fps) which is nearer."""
+        got = solve_crisp(30, 100)
+        assert got.pixels_per_second == pytest.approx(33.333, abs=0.01)
+        assert got.pixels_per_frame == 1
+
+    def test_does_not_chase_a_jumpy_exact_match(self):
+        """45 -> 50 (1px, smooth) beats 42.9 (3px at 14fps)."""
+        assert solve_crisp(45, 100).pixels_per_frame == 1
+
+    def test_allows_a_two_pixel_step_when_it_is_clearly_closer(self):
+        """60 -> 66.7 (2px at 33fps) rather than 50 (1px) which is 17% slow."""
+        got = solve_crisp(60, 100)
+        assert got.pixels_per_second == pytest.approx(66.667, abs=0.01)
+        assert got.pixels_per_frame == 2
+
+    def test_result_is_always_on_the_ladder(self):
+        ladder = {round(c.pixels_per_second, 3) for c in crisp_ladder(100)}
+        for target in range(5, 205, 5):
+            assert round(solve_crisp(target, 100).pixels_per_second, 3) in ladder
+
+    def test_adapts_to_a_slower_panel(self):
+        got = solve_crisp(30, 60)
+        assert got.pixels_per_second == pytest.approx(30.0)
+        assert got.pixels_per_frame == 1
+        assert got.frame_hold == 2
+
+    def test_speeds_beyond_the_panel_clamp_to_the_fastest_available(self):
+        got = solve_crisp(10_000, 100)
+        assert got.frame_hold == 1
+        assert got.pixels_per_frame == MAX_PIXELS_PER_FRAME
+
+    def test_steppiness_labels_are_sane(self):
+        assert solve_crisp(100, 100).steppiness == "smooth"
+        assert crisp_ladder(100)[0].steppiness == "stepped"