fix(fonts): load 4x6 on its pixel grid, from any working directory (#565)

* fix(fonts): load 4x6 on its pixel grid, from any working directory

`extra_small_font` loaded 4x6-font.ttf at 6, off the face's 7px grid.
Under `draw.fontmode = "1"` the mono rasteriser thresholds each glyph at
50% coverage, so every glyph lost its fourth column and deformed:
christmas-countdown rendered "UNTIL" as "VM1JL". The advance is 5px at
both sizes, so snapping to 7 reflows nothing.

- Sizes in DisplayManager._load_fonts go through crisp_size() instead of
  literals. crisp_size / FONT_PIXEL_GRID / FONT_NAME_ALIASES move to
  src/common/font_layout.py; sports_card re-exports them.
- Mirror the fix in VisualTestDisplayManager, the harness's fork of
  _load_fonts. Without it every golden is blessed at the old size.
- Resolve bundled font paths against the install root, not the cwd.
  FontManager._resolve_asset_path now delegates to
  font_layout.resolve_asset_path (kept by name; plugins probe for it).
- The startup banner's middle rung snaps to 7; the 5 rung stays off-grid
  on purpose (the only size that fits a dotted quad on 64px).
- loading.py reads all plugin JSON as UTF-8 (cp1252 on Windows aborted
  check_plugin.py on a 0x9d byte).
- check_plugin.py reports in ASCII and never dies on an unencodable char.

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

* fix(fonts): resolve relative asset paths from the install root, not the cwd

resolve_asset_path checked os.path.exists(relative_path) unconditionally,
so a relative asset path was still resolved against the process cwd first
-- exactly the dependency this module exists to remove. An unrelated
working directory that happens to contain assets/fonts/4x6-font.ttf (a
stale checkout, a copied assets folder, another project) would shadow the
real bundled font instead of the install root ever being consulted.

Only an absolute path is now returned as-is; a relative path always
resolves against _INSTALL_ROOT first, matching the docstring's stated
contract. FontManager._resolve_asset_path delegates to this function, so
it's covered by the same fix.

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

---------

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

src/common/font_layout.py

@@ -20,11 +20,23 @@ with on an LED panel.
 
 Use :func:`load_truetype` in place of ``ImageFont.truetype`` anywhere the
 result is drawn to a panel or compared against a golden image.
+
+The module also owns the other two things that decide whether a bundled face
+renders reproducibly, for the same reason — they are properties of the font
+file, not of whoever is drawing with it:
+
+* :func:`crisp_size` and :data:`FONT_PIXEL_GRID` — the size each face renders
+  on whole pixels at. ``4x6-font.ttf`` has a 7px grid, which is why the 6 that
+  reads as its natural size is the wrong number everywhere it appears.
+* :func:`resolve_asset_path` — ``assets/fonts/...`` resolved against the
+  install root rather than the process cwd.
 """
 
 from __future__ import annotations
 
-from typing import Any, Union
+import os
+from pathlib import Path
+from typing import Any, Dict, Union
 
 from PIL import ImageFont
 
@@ -41,3 +53,74 @@ def load_truetype(font: Union[str, Any], size: int, **kwargs: Any) -> ImageFont.
     """
     kwargs.setdefault("layout_engine", LAYOUT_ENGINE)
     return ImageFont.truetype(font, size, **kwargs)
+
+
+# --------------------------------------------------------------------------
+# Bundled-asset path resolution
+# --------------------------------------------------------------------------
+
+#: The install root, derived from this module's own location
+#: (``<root>/src/common/font_layout.py``) rather than from the process cwd.
+_INSTALL_ROOT = Path(__file__).resolve().parents[2]
+
+
+def resolve_asset_path(relative_path: str) -> str:
+    """Resolve a repo-relative asset path independently of the process cwd.
+
+    Prefers the path as given — so an absolute path is returned untouched and
+    behaviour is unchanged wherever the cwd already happened to be the install
+    root — then the install root derived above, then the original string so a
+    caller that wants to raise and fall back still can.
+
+    Without the fallback, any process started outside the install root (the
+    plugin safety harness, a manual ``python run.py`` from ``$HOME``, a unit
+    file written without ``WorkingDirectory``) silently loses every font and
+    degrades to PIL's default face.
+    """
+    if os.path.isabs(relative_path) and os.path.exists(relative_path):
+        return relative_path
+    candidate = _INSTALL_ROOT / relative_path
+    if candidate.exists():
+        return str(candidate)
+    return relative_path
+
+
+# --------------------------------------------------------------------------
+# Pixel-grid snapping
+# --------------------------------------------------------------------------
+
+#: Family aliases the web UI may write, mapped to the shipped filename.
+FONT_NAME_ALIASES: Dict[str, str] = {
+    "press_start": "PressStart2P-Regular.ttf",
+    "four_by_six": "4x6-font.ttf",
+}
+
+#: Pixel grid each face renders crisply on. Off-grid sizes anti-alias, which
+#: on an LED matrix is a dim lamp rather than a soft edge — and worse under
+#: ``draw.fontmode = "1"``, where the mono rasteriser thresholds each glyph at
+#: 50% coverage: an off-grid 4x6 glyph renders 3px wide instead of 4, so W/M
+#: and 0/8 stop being distinguishable. Off-grid sizes also make ``getlength``
+#: return a FreeType-dependent fractional advance, which is how two panels on
+#: one config centre the same string differently.
+FONT_PIXEL_GRID: Dict[str, int] = {
+    "PressStart2P-Regular.ttf": 8,
+    "4x6-font.ttf": 7,
+}
+
+
+def crisp_size(font_file, desired, aliases=None, grid_table=None):
+    """Snap *desired* to the nearest size *font_file* renders crisply at.
+
+    A face with no known grid is returned unchanged, so a user-supplied font is
+    never second-guessed.
+
+    ``aliases`` and ``grid_table`` default to the shared tables; a plugin that
+    ships an extra face can pass its own without forking this.
+    """
+    aliases = FONT_NAME_ALIASES if aliases is None else aliases
+    grid_table = FONT_PIXEL_GRID if grid_table is None else grid_table
+    font_file = aliases.get(font_file, font_file)
+    grid = grid_table.get(font_file)
+    if not grid or not desired or desired <= 0:
+        return desired
+    return max(grid, int(round(float(desired) / grid)) * grid)