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

web_interface/blueprints/api_v3.py

@@ -838,6 +838,10 @@ def save_main_config():
                 except (ValueError, TypeError):
                     return jsonify({'status': 'error', 'message': f"Invalid multiplexing value '{data['multiplexing']}'. Must be an integer from 0 to 22."}), 400
 
+            # Validate pixel_mapper_config (free-form mapper string, e.g. "U-mapper;Rotate:90")
+            if 'pixel_mapper_config' in data and not isinstance(data['pixel_mapper_config'], str):
+                return jsonify({'status': 'error', 'message': 'pixel_mapper_config must be a string (e.g. "U-mapper;Rotate:90" or empty)'}), 400
+
             # Validate row_address_type
             if 'row_address_type' in data:
                 try: