feat(mockups): slot 13 curtain call; allow three.js in 13

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

docs/DECISIONS.md

@@ -48,3 +48,8 @@ merge request. Both override the superpowers default paths.
 the repo every session — measurably slower and more tokens).
 **Consequences:** Rule changes happen in one file; pointer files must stay thin. Plans are working
 artefacts: kept after merge they go stale and contradict the code, so the spec is the record.
+
+## 2026-10-30 — three.js allowed in mockup 13
+
+**Decision:** mockup 13 (Curtain call) may use three.js, like 06 and 07. It mocks the frieze → 3D sea →
+frieze → cave transition, which needs the 3D sea. three.js is already a dependency; nothing new is added.