Add skin system: user-installable visual overlays for sports scoreboards (#419)

* Add skin system: user-installable visual overlays for sports scoreboards

Skins restyle a scoreboard's live/recent/upcoming rendering while the
host plugin keeps doing data fetching, scheduling, caching, live
priority, and vegas mode — the anti-fork alternative for users who only
want a different layout.

- src/skin_system/: ScoreboardSkin API, SkinContext (canvas + adaptive
  layout + logo/font helpers), discovery/loading runtime with API major
  version gating and per-skin module namespacing
- src/base_classes/sports.py: _render_game() seam at the three
  _draw_scorebug_layout call sites; skin-first with built-in fallback,
  3-strikes session disable, slow-render warning; per-mode skin config
- scripts/validate_skin.py: headless multi-mode/multi-size validator
  with bundled per-sport fixtures (no hardware or network needed)
- skins/example-classic-baseball/: working reference skin
- Web UI: served-schema Visual Skin dropdown (validation never
  enum-restricted, so uninstalled skins can't invalidate configs) and
  GET /api/v3/skins
- Store: registry entries with type "skin" install to skins/
- docs/SKIN_SYSTEM.md (architecture), docs/CREATING_SKINS.md (author
  guide incl. Claude Code prompt), view-model contract locked by tests

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LrCusPasy1qeUN5anK3aA1

* Address review feedback on skin system

- skin_runtime: cache the entry module so the 2nd/3rd load of the same
  skin (live/recent/upcoming hosts) doesn't re-execute it with unbound
  sibling aliases; rebind cached sibling modules to their bare names
  around entry import and restore prior bindings after; include per-
  manifest mtimes in the discovery cache fingerprint so in-place skin
  updates are picked up
- sports.py: count render_skin_card exceptions toward the 3-strike
  session disable
- store_manager: validate skin ids (pattern + resolved-path containment
  in skins/), reject registry/manifest id mismatches, and stage+validate
  downloads in a temp sibling before replacing an existing skin
- schema_manager: leave the schema untouched when the configured skin
  value is a per-mode mapping (a string dropdown could overwrite it)
- validate_skin.py: reject non-positive sizes and non-object --options
  at parse time; support --output-dir outside the repo; type annotations
- example skin: validate accent_color once at load with logged fallback
- fixtures: pregame 0-0 scores in football/hockey upcoming fixtures
- api /skins: rely on the self-invalidating discovery cache instead of
  force_refresh
- docs: valid JSON manifest example, load_logo caching semantics spelled
  out, language ids on fenced blocks
- tests: view-model contract test now exercises the real extractor;
  regression test for repeated same-skin loads with sibling modules

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LrCusPasy1qeUN5anK3aA1

---------

Co-authored-by: Claude <noreply@anthropic.com>

README.md

@@ -440,6 +440,16 @@ See the [Plugin Store documentation](https://github.com/ChuckBuilds/ledmatrix-pl
 
 For plugin development, check out the [Hello World Plugin](https://github.com/ChuckBuilds/ledmatrix-hello-world) repository as a starter template.
 
+### Visual Skins for Scoreboards
+
+Want a different look for a sports scoreboard without forking the plugin?
+**Skins** restyle the live/recent/upcoming screens while the plugin keeps
+handling data, scheduling, caching, and vegas mode. Install one with
+`git clone <skin repo> skins/<skin-id>`, select it in the plugin's config,
+and you're done — see [docs/SKIN_SYSTEM.md](docs/SKIN_SYSTEM.md) (how it
+works) and [docs/CREATING_SKINS.md](docs/CREATING_SKINS.md) (build your own,
+including a ready-made Claude Code prompt).
+
 2. **Built-in Managers Deprecated**: The built-in managers (hockey, football, stocks, etc.) are now deprecated and have been moved to the plugin system. **You must install replacement plugins from the Plugin Store** in the web interface instead. The plugin system provides the same functionality with better maintainability and extensibility.
 </details>