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

test/test_doc_links.py

@@ -6,20 +6,23 @@ links inside fenced code blocks.
 """
 import re
 from pathlib import Path
+from typing import Iterator
 
 PROJECT_ROOT = Path(__file__).resolve().parent.parent
 LINK_RE = re.compile(r'\[[^\]]*\]\(([^)\s]+)\)')
 FENCE_RE = re.compile(r'^(```|~~~)')
 
 
-def _md_files():
+def _md_files() -> Iterator[Path]:
+    """Yield active markdown files (repo root + docs/, excluding docs/archive/)."""
     yield from PROJECT_ROOT.glob('*.md')
     for path in PROJECT_ROOT.glob('docs/**/*.md'):
         if 'archive' not in path.parts:
             yield path
 
 
-def test_relative_markdown_links_resolve():
+def test_relative_markdown_links_resolve() -> None:
+    """Every relative markdown link outside code fences must resolve on disk."""
     broken = []
     for md in _md_files():
         in_fence = False