docs: address CodeRabbit review comments on #306

Reviewed all 12 CodeRabbit comments on PR #306, verified each against
the current code, and fixed the 11 valid ones. The 12th finding is a
real code bug (cache_manager.delete() calls in api_helper.py and
resource_monitor.py) that's already in the planned follow-up code-fix
PR, so it stays out of this docs PR.

Fixed:

.cursor/plugins_guide.md, .cursor/README.md, .cursorrules
- I claimed "there is no --emulator flag" in 3 places. Verified in
  run.py:19-20 that the -e/--emulator flag is defined and functional
  (it sets os.environ["EMULATOR"]="true" before the display imports).
  Other docs I didn't touch (.cursor/plugin_templates/QUICK_START.md,
  docs/PLUGIN_DEVELOPMENT_GUIDE.md) already use the flag correctly.
  Replaced all 3 wrong statements with accurate guidance that
  both forms work and explains the CLI flag's relationship to the
  env var.

.cursorrules, docs/GETTING_STARTED.md, docs/WEB_INTERFACE_GUIDE.md,
docs/PLUGIN_DEVELOPMENT_GUIDE.md
- Four places claimed "the plugin loader also falls back to plugins/".
  Verified that PluginManager.discover_plugins()
  (src/plugin_system/plugin_manager.py:154) only scans the
  configured directory — no fallback. The fallback to plugins/
  exists only in two narrower places: store_manager.py:1700-1718
  (store install/update/uninstall operations) and
  schema_manager.py:70-80 (schema lookup for the web UI form
  generator). Rewrote all four mentions with the precise scope.
  Added a recommendation to set plugin_system.plugins_directory
  to "plugins" for the smoothest dev workflow with
  dev_plugin_setup.sh symlinks.

docs/FONT_MANAGER.md
- The "Status" warning told plugin authors to use
  display_manager.font_manager.resolve_font(...) as a workaround for
  loading plugin fonts. Verified in src/font_manager.py that
  resolve_font() takes a family name, not a file path — so the
  workaround as written doesn't actually work. Rewrote to tell
  authors to load the font directly with PIL or freetype-py in their
  plugin.
- The same section said "the user-facing font override system in the
  Fonts tab still works for any element that's been registered via
  register_manager_font()". Verified in
  web_interface/blueprints/api_v3.py:5404-5428 that
  /api/v3/fonts/overrides is a placeholder implementation that
  returns empty arrays and contains "would integrate with the actual
  font system" comments — the Fonts tab does not have functional
  integration with register_manager_font() or the override system.
  Removed the false claim and added an explicit note that the tab
  is a placeholder.

docs/ADVANCED_FEATURES.md:523
- The on-demand section said REST/UI calls write a request "into the
  cache manager (display_on_demand_config key)". Wrong — verified
  via grep that api_v3.py:1622 and :1687 write to
  display_on_demand_request, and display_on_demand_config is only
  written by the controller during activation
  (display_controller.py:1195, cleared at :1221). Corrected the key
  name and added controller file:line references so future readers
  can verify.

docs/ADVANCED_FEATURES.md:803
- "Plugins using the background service" paragraph listed all
  scoreboard plugins but an orphaned "⏳ MLB (baseball)" bullet
  remained below from the old version of the section. Removed the
  orphan and added "baseball/MLB" to the inline list for clarity.

web_interface/README.md
- The POST /api/v3/system/action action list was incomplete. Verified
  in web_interface/app.py:1383,1386 that enable_autostart and
  disable_autostart are valid actions. Added both.
- The Plugin Store section was missing
  GET /api/v3/plugins/store/github-status (verified at
  api_v3.py:3296). Added it.
- The SSE line-range reference was app.py:607-615 but line 619
  contains the "Exempt SSE streams from CSRF and add rate limiting"
  block that's semantically part of the same feature. Extended the
  range to 607-619.

docs/GETTING_STARTED.md
- Rows/Columns step said "Columns: 64 or 96 (match your hardware)".
  The web UI's validation accepts any integer in 16-128. Clarified
  that 64 and 96 are the common bundled-hardware values but the
  valid range is wider.

Not addressed (out of scope for docs PR):

- .cursorrules:184 CodeRabbit comment flagged the non-existent
  cache_manager.delete() calls in src/common/api_helper.py:287 and
  src/plugin_system/resource_monitor.py:343. These are real CODE
  bugs, not doc bugs, and they're the first item in the planned
  post-docs-refresh code-cleanup PR (see
  /home/chuck/.claude/plans/warm-imagining-river.md). The docs in
  this PR correctly state that delete() doesn't exist on
  CacheManager — the fix belongs in the follow-up code PR that
  either adds a delete() shim or updates the two callers.

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

.cursor/plugins_guide.md

@@ -403,7 +403,10 @@ cd /path/to/LEDMatrix
 2. **Test with the dev preview server**:
    `python3 scripts/dev_server.py` (then open `http://localhost:5001`).
    Or run the full display in emulator mode with
-   `EMULATOR=true python3 run.py`. There is no `--emulator` flag.
+   `python3 run.py --emulator` (or equivalently
+   `EMULATOR=true python3 run.py`). The `-e`/`--emulator` CLI flag is
+   defined in `run.py:19-20` and sets the same `EMULATOR` environment
+   variable internally.
 3. **Check logs** for errors or warnings
 4. **Update configuration** in `config/config.json` if needed
 5. **Iterate** until plugin works correctly