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>

CHANGELOG.md

@@ -33,6 +33,12 @@ accepts both, but the store flags the old spelling as deprecated
   its own default (a failed lookup is retried after 30 minutes). Clearing an
   app's location in the web UI now actually clears it; the save used to drop
   the blank field, so the old value stayed.
+- `src.common.bdf_font` — `load_bdf_face(path, size)` (a cached
+  `freetype.Face` plus the pixel size it really renders at, falling back to
+  the file's native strike) and `draw_bdf_text(draw, text, x, y, face, color)`.
+  `DisplayManager`, `FontManager`, `element_style` and the plugin test harness
+  now all load and draw BDF text through it; the panel's pixels are unchanged
+  and BDF text draws 10-250x faster.
 
 - The web UI's Fonts tab has a **Used by** column: the loaded plugins that
   registered each font with `FontManager.register_manager_font()`, published