Story 4.2: Implement full AV replacement with WebRTC stream access

- Update FoundryAdapter to properly detect and expose WebRTC stream access
- Modify ScryingPoolStrip to create video elements with WebRTC streams
- Add video container to roster-strip.hbs template with conditional rendering
- Add CSS to hide Foundry's AV dock (#av and .camera-view)
- Add CSS styling for video containers and elements
- Fix unused variable in FoundryAdapter.buildWebRTCSurface
- Add comprehensive test script for stream access implementation

Architecture: Full replacement mode where module hides Foundry's AV dock
and creates its own video elements using game.webrtc.client.getMediaStreamForUser()

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

templates/roster-strip.hbs

@@ -44,7 +44,12 @@
                   aria-label="{{name}} — {{stateLabel}}"
                   aria-pressed="false">
 
-            {{!-- Avatar image (32px rounded) --}}
+            {{!-- Video container for stream-access mode (full AV replacement) --}}
+            {{#if hasStreamAccess}}
+              <div class="sp-participant-video" aria-hidden="true"></div>
+            {{/if}}
+
+            {{!-- Avatar image (32px rounded) - shown as fallback when no video --}}
             <img class="sp-avatar__img" src="{{avatarSrc}}" alt="" aria-hidden="true" />
 
             {{!-- Corner badge (12px bottom-right) --}}