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

scripts/install_dependencies_apt.py

@@ -44,7 +44,7 @@ def install_via_apt(package_name: str) -> Tuple[bool, str]:
     apt_package_map = {
         'flask': 'python3-flask',
         'PIL': 'python3-pil',
-        'freetype': 'python3-freetype',
+        'freetype-py': 'python3-freetype',
         'psutil': 'python3-psutil',
         'werkzeug': 'python3-werkzeug',
         'numpy': 'python3-numpy',
@@ -95,7 +95,9 @@ def install_via_pip(package_name: str) -> Tuple[bool, str]:
 
 
 # Distribution (pip/apt) names whose importable module name differs.
-IMPORT_NAME_MAP = {}
+IMPORT_NAME_MAP = {
+    'freetype-py': 'freetype',
+}
 
 
 def check_package_installed(package_name: str) -> bool:
@@ -139,7 +141,7 @@ def main():
     required_packages = [
         'flask',
         'PIL',
-        'freetype',
+        'freetype-py',
         'psutil',
         'werkzeug',
         'numpy',