fix: address CodeRabbit review findings

Verified each finding against the code; fixes for the valid ones:

- install_dependencies_apt.py: the installer listed 'freetype', but the
  declared dependency is freetype-py — an apt miss would pip-install the
  wrong PyPI package. Now installs freetype-py with an import-name
  mapping (pre-existing bug, surfaced by the review).
- api_v3.py: pixel_mapper_config is validated as a string before being
  saved to display.hardware (JSON callers could previously store an
  object/list the matrix library can't use).
- .cursorrules: the Plugin Loading Process and File Organization
  sections still said discovery scans plugins/ — now consistent with the
  corrected overview (configured directory, default plugin-repos/).
- README.md: removed the stale '(except the core calendar)' claim — no
  core calendar exists in src/ — and qualified the plugin inventory
  (official plugins in the monorepo; third-party from their own repos).
- CONFIG_REFERENCE.md: hardware_mapping now shows the code fallback
  (adafruit-hat-pwm) alongside the template value.
- PLUGIN_REGISTRY_SETUP_GUIDE.md: check_plugin.py takes --plugin, not a
  positional id.
- scripts/fix_perms/fix_*.sh: exec bits set so the documented
  'sudo ./...' invocations work.
- Guard tests hardened: template guard now catches multi-line
  render_template() calls; widget guard parses actual <script> src
  values and fails if the widgets dir goes missing; type hints and
  docstrings added per repo coding guidelines.

Skipped with reasons (noted on the PR): limit_refresh_rate_hz 100-vs-90
is documented as intentional in CONFIG_REFERENCE.md; the psutil comment
already names the enforcing manifest; docs/archive/ findings are out of
scope per the docs policy (archive may rot).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SXb4mKcAkVaxkeTb3YnAdr

README.md

@@ -54,10 +54,11 @@ LEDMatrix is a plugin platform: the displays below are plugins installed
 from the built-in Plugin Store (web interface → Plugins), where each can be
 individually enabled, ordered, and configured — display durations, teams,
 stocks, weather, timezones, and more. The core repo ships with just two
-bundled plugins (`starlark-apps` and `web-ui-info`); everything else lives
-in the [ledmatrix-plugins](https://github.com/ChuckBuilds/ledmatrix-plugins)
-monorepo and installs with one click. Displays available in the store
-include:
+bundled plugins (`starlark-apps` and `web-ui-info`); the official plugins
+live in the [ledmatrix-plugins](https://github.com/ChuckBuilds/ledmatrix-plugins)
+monorepo and install with one click, and third-party plugins can be
+installed from their own GitHub repositories. Displays available in the
+store include:
 
 ### Time and Weather
 - Real-time clock display (2x 64x32 Displays 4mm Pixel Pitch)
@@ -428,7 +429,7 @@ I recommend using the web-ui "Quick Actions" to control the Display.
 ## Plugins
 
 <details>
-LEDMatrix uses a plugin-based architecture where all display functionality (except the core calendar) is implemented as plugins. All managers that were previously built into the core system are now available as plugins through the Plugin Store.
+LEDMatrix uses a plugin-based architecture where all display functionality is implemented as plugins. All managers that were previously built into the core system are now available as plugins through the Plugin Store.
 
 ### Plugin Store
 See the [Plugin Store documentation](https://github.com/ChuckBuilds/ledmatrix-plugins) for detailed installation instructions.