fix(fonts): unloading a plugin forgets its manifest fonts (#757)

* fix(fonts): unloading a plugin forgets its manifest fonts

PluginManager.unload_plugin() and the failed-load cleanup only called
FontManager.forget_manager_fonts(), which drops usage data. The plugin's
manifest registrations stayed: plugin_fonts / plugin_font_catalogs, its
plugin_id::family entries in font_catalog, and cached font objects for them
-- so its fonts kept resolving after unload and a family a reinstalled
manifest dropped stayed registered. The deprecated unregister_plugin_fonts
did this cleanup but nothing called it; it was removed in #708.

Add FontManager.forget_plugin_fonts(plugin_id) and call it from both
paths alongside forget_manager_fonts (each guarded on its own, so a font
manager stub with only one still works). FontManager takes no locks, so
like forget_manager_fonts it uses atomic pops over snapshots. A reload
(unload + load) registers the manifest fonts again and they resolve.

Raised by CodeRabbit on #709.

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

* fix(fonts): call the font manager's forget methods without a None-able local

Pylint E1102 (Codacy) read getattr(..., None) as possibly not callable.

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

---------

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

docs/FONT_MANAGER.md

@@ -203,6 +203,7 @@ Current methods:
 | `measure_text(text, font)` | `(width, height, baseline)` |
 | `get_font_height(font)` | Line height |
 | `register_plugin_fonts(plugin_id, font_manifest)` | Register a plugin's fonts (core calls it at load) |
+| `forget_plugin_fonts(plugin_id)` | Drop a plugin's manifest fonts and their cached objects (core calls it when a plugin unloads) |
 | `clear_cache()` | Drop cached fonts and metrics |
 | `font_catalog` (attribute) | Family name → file path |
 
@@ -218,5 +219,6 @@ Removed in 3.8.0, after logging a deprecation warning on first call since
 | `get_performance_stats()` | — |
 | `set_override()`, `remove_override()`, `get_overrides()` | a font field in your plugin's config schema |
 | `get_manager_fonts()`, `get_detected_fonts()` | — |
-| `get_plugin_fonts()`, `unregister_plugin_fonts()` | — |
+| `get_plugin_fonts()` | — |
+| `unregister_plugin_fonts()` | `forget_plugin_fonts()` (core calls it on unload) |
 | `add_font()`, `remove_font()`, `validate_font()` | the web UI's Fonts tab |