docs: flag aspirational/regressed features in plugin docs

These docs describe features that exist as documented in the doc but
either never wired up or regressed when v3 shipped. Each gets a clear
status banner so plugin authors don't waste time chasing features that
don't actually work.

FONT_MANAGER.md
- The "For Plugin Developers / Plugin Font Registration" section
  documents adding a "fonts" block to manifest.json that gets
  registered via FontManager.register_plugin_fonts(). The method
  exists at src/font_manager.py:150 but is **never called from
  anywhere** in the codebase (verified: zero callers). A plugin
  shipping a manifest "fonts" block has its fonts silently ignored.
  Added a status warning and a note about how to actually ship plugin
  fonts (regular files in the plugin dir, loaded directly).

PLUGIN_IMPLEMENTATION_SUMMARY.md
- Added a top-level status banner.
- Architecture diagram referenced src/plugin_system/registry_manager.py
  (which doesn't exist) and listed plugins/ as the install location.
  Replaced with the real file list (plugin_loader, schema_manager,
  health_monitor, operation_queue, state_manager) and pointed at
  plugin-repos/ as the default install location.
- "Dependency Management: Virtual Environments" — verified there's no
  per-plugin venv. Removed the bullet and added a note that plugin
  Python deps install into the system Python environment, with no
  conflict resolution.
- "Permission System: File Access Control / Network Access /
  Resource Limits / CPU and memory constraints" — none of these
  exist. There's a resource_monitor.py and health_monitor.py for
  metrics/warnings, but no hard caps or sandboxing. Replaced the
  section with what's actually implemented and a clear note that
  plugins run in the same process with full file/network access.

PLUGIN_CUSTOM_ICONS.md and PLUGIN_CUSTOM_ICONS_FEATURE.md
- The custom-icon feature was implemented in the v2 web interface
  via a getPluginIcon() helper in templates/index_v2.html that read
  the manifest "icon" field. When the v3 web interface was built,
  that helper wasn't ported. Verified in
  web_interface/templates/v3/base.html:515 and :774, plugin tab
  icons are hardcoded to `fas fa-puzzle-piece`. The "icon" field in
  plugin manifests is currently silently ignored (verified with grep
  across web_interface/ and src/plugin_system/ — zero non-action-
  related reads of plugin.icon or manifest.icon).
- Added a status banner to both docs noting the regression so plugin
  authors don't think their custom icons are broken in their own
  plugin code.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/PLUGIN_CUSTOM_ICONS.md

@@ -1,5 +1,16 @@
 # Plugin Custom Icons Guide
 
+> ⚠️ **Status:** the `icon` field in `manifest.json` is currently
+> **not honored by the v3 web interface**. Plugin tab icons are
+> hardcoded to `fas fa-puzzle-piece` in
+> `web_interface/templates/v3/base.html:515` and `:774`. The icon
+> field was originally read by a `getPluginIcon()` helper in the v2
+> templates, but that helper wasn't ported to v3. Setting `icon` in a
+> manifest is harmless (it's just ignored) so plugin authors can leave
+> it in place for when this regression is fixed.
+>
+> Tracking issue: see the LEDMatrix repo for the open ticket.
+
 ## Overview
 
 Plugins can specify custom icons that appear next to their name in the web interface tabs. This makes your plugin instantly recognizable and adds visual polish to the UI.