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>

CHANGELOG.md

@@ -21,6 +21,14 @@ accepts both, but the store flags the old spelling as deprecated
 
 ### Fixed
 
+- Unloading a plugin now forgets the fonts its manifest registered, not only
+  the fonts it reported using. Its `plugin_id::family` entries kept resolving
+  and their cached font objects stayed alive until a restart, and a family a
+  reinstalled plugin's manifest dropped stayed registered. The new
+  `FontManager.forget_plugin_fonts(plugin_id)` does the cleanup;
+  `PluginManager.unload_plugin()` and a failed load call it alongside
+  `forget_manager_fonts()`, and a reload registers the manifest's fonts again.
+
 - The web preview and `/api/v3/display/current` no longer stay black for a
   whole screen that draws its card once and then holds it. The snapshot is
   written from `update_display()` at most once per write interval, so a frame