feat(vegas): one declared participation per plugin (scroll | pause | exclude) (#682)

A plugin takes part in Vegas mode in one declared way: 'scroll', 'pause'
or 'exclude', resolved from the user's vegas_participation setting, the
manifest field, then the legacy hooks, so no plugin changes behaviour.
The stream manager decides inclusion and pauses through it; the installed
plugins API and the Vegas plugin-order list report it. Deprecates
get_supported_vegas_modes, get_vegas_segment_width and vegas_panel_count
for removal in 3.9.0, and regenerates docs/DEPRECATIONS_3.8.md to include
them.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/vegas_mode/plugin_adapter.py

@@ -1369,29 +1369,6 @@ class PluginAdapter:
             logger.debug("[%s] Cleared plugin scroll cache", plugin_id)
         return cleared
 
-    def get_content_type(self, plugin: 'BasePlugin', plugin_id: str) -> str:
-        """
-        Get the type of content a plugin provides.
-
-        Args:
-            plugin: Plugin instance
-            plugin_id: Plugin identifier
-
-        Returns:
-            'multi' for multiple items, 'static' for single frame, 'none' for excluded
-        """
-        if hasattr(plugin, 'get_vegas_content_type'):
-            try:
-                return plugin.get_vegas_content_type()
-            except (AttributeError, TypeError, ValueError):
-                logger.exception(
-                    "Error calling get_vegas_content_type() on %s",
-                    plugin_id
-                )
-
-        # Default to static for plugins without explicit type
-        return 'static'
-
     def cleanup(self) -> None:
         """Clean up resources."""
         with self._cache_lock: