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

docs/CONFIG_REFERENCE.md

@@ -55,7 +55,7 @@ in `DisplayManager` (`src/display_manager.py`, ~lines 270–295).
 | `chain_length` | int, `2` |
 | `parallel` | int, `1` |
 | `brightness` | int, `90` |
-| `hardware_mapping` | string, `"adafruit-hat"` |
+| `hardware_mapping` | string, `"adafruit-hat"` (code default `"adafruit-hat-pwm"`) |
 | `scan_mode` | int, `0` |
 | `pwm_bits` | int, `9` (code default 10) |
 | `pwm_dither_bits` | int, `1` |

docs/PLUGIN_REGISTRY_SETUP_GUIDE.md

@@ -333,7 +333,7 @@ python3 scripts/setup_plugin_repos.py
 python3 scripts/audit_plugins.py
 
 # Validate a single plugin
-python3 scripts/check_plugin.py <plugin-id>
+python3 scripts/check_plugin.py --plugin <plugin-id>
 ```
 
 Registry regeneration (`update_registry.py`) lives in the