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>

src/skin_system/fixtures/baseball_recent.json

@@ -0,0 +1,39 @@
+{
+  "id": "401570001",
+  "game_time": "7:05PM",
+  "game_date": "Jul 16th",
+  "start_time_utc": "2026-07-16T23:05:00+00:00",
+  "status_text": "Final",
+  "is_live": false,
+  "is_final": true,
+  "is_upcoming": false,
+  "is_halftime": false,
+  "is_period_break": false,
+  "home_abbr": "LAD",
+  "home_id": "19",
+  "home_score": "5",
+  "home_logo_path": "src/skin_system/fixtures/placeholder_home.png",
+  "home_logo_url": null,
+  "home_record": "58-33",
+  "away_abbr": "SF",
+  "away_id": "26",
+  "away_score": "3",
+  "away_logo_path": "src/skin_system/fixtures/placeholder_away.png",
+  "away_logo_url": null,
+  "away_record": "49-42",
+  "is_within_window": true,
+  "status": "STATUS_FINAL",
+  "status_state": "post",
+  "inning": 9,
+  "inning_half": "top",
+  "balls": 0,
+  "strikes": 0,
+  "outs": 3,
+  "bases_occupied": [
+    false,
+    false,
+    false
+  ],
+  "start_time": "2026-07-16T23:05:00Z",
+  "series_summary": "Series tied 2-2"
+}