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/FONT_MANAGER.md

@@ -138,7 +138,21 @@ font = self.font_manager.resolve_font(
 
 ## For Plugin Developers
 
-### Plugin Font Registration
+> ⚠️ **Status**: the plugin-font registration described below is
+> implemented in `src/font_manager.py:150` (`register_plugin_fonts()`)
+> but is **not currently wired into the plugin loader**. Adding a
+> `"fonts"` block to your plugin's `manifest.json` will silently have
+> no effect — the FontManager method exists but nothing calls it.
+>
+> Until that's connected, plugin authors should ship custom fonts as
+> regular files inside the plugin directory (e.g., `assets/myfont.ttf`)
+> and reference them by relative path from the plugin's `manager.py`
+> via `display_manager.font_manager.resolve_font(...)` or by loading
+> with PIL directly. The user-facing font override system in the
+> **Fonts** tab still works for any element that's been registered via
+> `register_manager_font()`.
+
+### Plugin Font Registration (planned)
 
 In your plugin's `manifest.json`: