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>

docs/PLUGIN_API_REFERENCE.md

@@ -36,7 +36,10 @@ self.enabled            # Boolean enabled status
 
 #### `update() -> None`
 
-Fetch/update data for this plugin. Called based on `update_interval` specified in the plugin's manifest.
+Fetch/update data for this plugin. Called on the plugin's update interval:
+the value `get_update_interval()` returns when it returns a number, otherwise
+the `update_interval` in the plugin's manifest (see
+[`get_update_interval()`](#get_update_interval---optionalfloat) below).
 
 **Example**:
 ```python
@@ -109,6 +112,46 @@ Called when plugin is enabled.
 
 Called when plugin is disabled.
 
+#### `get_update_interval() -> Optional[float]`
+
+How often this plugin wants `update()` called right now, in seconds. The
+manifest's `update_interval` is one static number; override this when the
+right cadence depends on state only the plugin knows, e.g. poll every 15s
+while a game is live and fall back to the manifest value otherwise.
+
+**Returns**: seconds as a number, or `None` (the default) for no opinion.
+
+How `PluginManager` (`_get_plugin_update_interval` in
+`src/plugin_system/plugin_manager.py`) resolves the interval on each
+scheduling tick:
+
+1. It calls `get_update_interval()`. A number wins over everything below.
+   Values under `PluginManager.MIN_DYNAMIC_UPDATE_INTERVAL` (5 seconds) are
+   raised to it.
+2. If the hook returns `None`, raises, or returns something that isn't a
+   finite number (a `bool`, a string, NaN, infinity), it is ignored and the
+   static interval applies: the manifest's `update_interval`, else
+   `update_interval` in the plugin's section of `config.json`, else 60
+   seconds.
+
+The static value is cached per plugin until the plugin is loaded or
+unloaded again, so editing `update_interval` in config takes effect on the
+next reload. The hook's return value is never cached: it is called on every
+tick of the display loop, so keep it to attribute reads (no config lookups,
+no I/O, no locks a fetch might hold) and don't let it raise.
+
+**Example**:
+```python
+def get_update_interval(self):
+    # Fast while something is live, manifest default otherwise.
+    if any(m.live_games for m in self._live_managers):
+        return self.config.get("live_update_interval", 15)
+    return None
+```
+
+Added in core 3.4.0; older cores never call it, so a plugin that relies on
+it should floor `ledmatrix_min_version` at `3.4.0`.
+
 #### `get_display_duration() -> float`
 
 Get display duration for this plugin. Can be overridden for dynamic durations.