chore: mark skins unsupported, fix stale docs and preview size, prepare 3.4.0

Skins: no current scoreboard plugin builds on src.base_classes, so the only
skin hook (SportsCore._render_game) never runs. The plugin schema endpoint no
longer injects the Visual Skin dropdown, the store hides and refuses
"type": "skin" registry entries, and GET /api/v3/skins reports
supported: false with a message. Stored skin config still loads and saves.
src/skin_system/ and its tests are unchanged apart from the support flag.

Docs: check_plugin.py/render_plugin.py examples use --plugin; document
BasePlugin.get_update_interval() and its interaction with the manifest
update_interval; CLAUDE.md drops the stale template line number and
recommends display_manager.width/height.

Preview size: new src/display_geometry.py holds the size computation and
defaults DisplayManager uses (double-sided applied, chain_length default 2).
The web preview, /display/current, Starlark magnify default, sync handshake
and two dev scripts use it.

Release: __version__ 3.4.0, CHANGELOG 3.4.0 section plus a 3.3.0 tag note.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

scripts/dev/vegas_audit.py

@@ -72,12 +72,8 @@ def load_main_config(path: Path) -> Dict[str, Any]:
 
 def display_size_from_config(config: Dict[str, Any]) -> tuple:
     """Derive the logical ticker size the way DisplayManager does."""
-    hw = config.get('display', {}).get('hardware', {})
-    cols = int(hw.get('cols', 64))
-    chain = int(hw.get('chain_length', 1))
-    rows = int(hw.get('rows', 32))
-    parallel = int(hw.get('parallel', 1))
-    return cols * chain, rows * parallel
+    from src.display_geometry import logical_size
+    return logical_size(config)
 
 
 def enabled_plugin_ids(config: Dict[str, Any]) -> List[str]: