feat(career): bundle the arena venue pack (career stage 3) (#961)

Feedback Arena (150 stars) now ships in every build like the bar:
4 reactive crowd loops, 2 stingers, and a flyover intro rendered
from the UE5 arena scene (200 spectators + 396-body intro fill,
state-reactive rig lighting). Served by the existing bundled-pack
fallback; venues.json unchanged. Audio files are dive-bar
placeholders until arena-scale recordings land.

Largest file is 89MB — future re-renders must stay under GitHub's
100MB hard limit.

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

plugins/career/venue-packs/arena/manifest.json

@@ -0,0 +1,8 @@
+{
+    "venue": "arena",
+    "version": 1,
+    "loops": {"bored": "bored.mp4", "neutral": "neutral.mp4", "engaged": "engaged.mp4", "ecstatic": "ecstatic.mp4"},
+    "stingers": {"clap": "clap.mp4", "cheer": "cheer.mp4"},
+    "intro": {"video": "intro.mp4", "audio": "arena-ambience.mp3"},
+    "sfx": {"up": "sfx-up.mp3", "down": "sfx-down.mp3"}
+}