refactor(fonts): one BDF loader and one BDF rasterizer (#627)

* refactor(fonts): one BDF loader and one BDF rasterizer

BDF faces were loaded three ways (FontManager._load_bdf_font,
element_style._load_bdf, DisplayManager._load_fonts) and drawn by two
copies of the same per-pixel loop (DisplayManager._draw_bdf_text and the
plugin test harness's "replicated" copy), which golden images and
check_plugin/dev_server previews rely on matching the panel.

src/common/bdf_font.py now owns both:
- load_bdf_face(path, size) -> (face, realised_px): native-strike fallback
  for sizes the file lacks, one bounded LRU cache keyed on path, size and
  mtime. FontManager, element_style and DisplayManager delegate to it;
  read_bdf_native_size moves here (the old names delegate).
- draw_bdf_text(draw, text, x, y, face, color, clip): builds each glyph as
  a 1-bit mask and fills it with ImageDraw.bitmap instead of a draw.point
  per pixel. A blending Draw (RGB image, "RGBA" mode) keeps the point path
  so translucent colours still blend.

Pixel-identical: 220,032 renders (every bundled BDF at native and
off-strike sizes, 14 strings, 4 colours, clipped on every edge, through
each old loader x rasterizer) match origin/main byte for byte.
test/test_bdf_font.py keeps a lightweight version against a frozen copy of
the old loop. DisplayManager._draw_bdf_text goes from 1.4-23 ms to about
0.1 ms per string (the old loop re-read FreeType's buffer as a Python list
for every pixel).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* fix(testing): harness calendar_font is sized like the panel's

VisualTestDisplayManager built its 5x7 calendar_font / bdf_5x7_font as a
bare freetype.Face. With no size set its ascender reads 0, so BDF text
drawn with it landed 6px above where DisplayManager draws it -- entirely
off the canvas at y=0 -- and get_font_height() returned 0. Golden images
and check_plugin / dev_server previews showed text the panel does not.

Load it through load_bdf_face at the panel's 7px, so it is the very face
DisplayManager uses. Across the differential run this changes only the
cases drawn with the harness's own calendar_font (968 of 220,032), which
now match the panel's output.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* fix(fonts): one BDF face per thread

The shared face cache now hands every loader (FontManager, element_style,
DisplayManager, the harness) the same freetype.Face. FreeType does not allow
two threads to use one face at once, since load_char rewrites its glyph
slot, so key the cache by thread as well.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

src/common/README.md

@@ -36,6 +36,15 @@ Utilities for loading and managing team logos.
 
 Utilities for text processing and formatting.
 
+## BDF Fonts (`bdf_font.py`)
+
+The one way to load and draw BDF bitmap fonts. `load_bdf_face(path, size)`
+returns `(face, realised_px)`, falling back to the file's native strike when
+it has none at `size`; `draw_bdf_text(draw, text, x, y, face, color)` draws
+top-left anchored onto a PIL `ImageDraw` exactly as the panel does.
+`DisplayManager`, `FontManager`, `element_style` and the plugin test harness
+all go through it.
+
 ## Scroll Helpers (`scroll_helper.py`)
 
 Utilities for scrolling text on the display.