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>

schema/manifest_schema.json

@@ -149,6 +149,11 @@
       },
       "description": "Array of display mode names this plugin provides"
     },
+    "vegas_participation": {
+      "type": "string",
+      "enum": ["scroll", "pause", "exclude"],
+      "description": "How this plugin takes part in Vegas mode by default: 'scroll' (its content scrolls by), 'pause' (the scroll stops for its turn and display() draws it full screen) or 'exclude' (left out). A user's per-plugin vegas_participation setting overrides it. Omit it to derive the participation from get_vegas_display_mode() / get_vegas_content_type(). Cores before 3.8.0 ignore it."
+    },
     "api_requirements": {
       "type": "array",
       "items": {