feat(venue): reactive crowd video layer behind the 3D highway (career mode PR1)

Two crossfading video backdrop planes in the highway_3d venue background
style, driven by a new venue-crowd.js state machine that maps
v3:live-performance-state to crowd states (bored/neutral/engaged/ecstatic)
with 3s stability + 8s dwell hysteresis, plus one-shot reaction stingers
on streak milestones and end-of-song accuracy. Inert without a venue pack
manifest (career plugin, PR2) or the feedBack-venue-crowd-dev flag — the
static bg plate behaves exactly as before.

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

static/v3/venue-scene-3d.js

@@ -105,6 +105,17 @@
         setH3dMood(_lastMood);
         syncInstrumentPov();
         syncVenueMotion();
+        syncCrowd(true);
+    }
+
+    function syncCrowd(on) {
+        // Reactive crowd video layer (career mode) — inert without a pack.
+        try {
+            if (root && root.v3VenueCrowd &&
+                typeof root.v3VenueCrowd.setVenueActive === 'function') {
+                root.v3VenueCrowd.setVenueActive(!!on);
+            }
+        } catch (_) { /* visual-only */ }
     }
 
     function syncVenueMotion() {
@@ -128,6 +139,7 @@
         _assetsLoaded = false;
         _loadFailed = false;
         setH3dActive(false);
+        syncCrowd(false);
         syncPlaceholderVisibility();
     }