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

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

docs/design/002-direction-round-notes.md

@@ -184,6 +184,20 @@ This file exists because of that request. It gets updated after every user messa
 - *(Claude's proposal)* The 2D sea chart is now 12 Vellum atlas with threads. Next open questions are
   the look of the whole and how the frieze/3D layers attach to this chart.
 
+### 13. A short mockup of the transition
+
+*(Claude's question)* The look of the whole: 1 vellum throughout, 2 two worlds (vellum chart and 3D,
+terracotta frieze only at transitions; Claude's recommendation), 3 frieze-led. Offered a short
+mockup of the transition.
+
+> "Short mockup please :)"
+
+- The user did not pick 1/2/3 yet, so mockup **13 Curtain call** shows the transition sequence with
+  the look as a toggle (1/2/3), defaulting to 2.
+- Sequence: frieze (2D) → morphs into the 3D sea → ship reaches Cyclops' Island → back to the frieze
+  → into the cave. The vellum chart (12) appears as the map on the captain's table at sea.
+- three.js is allowed in 13 (it is already a dependency; it reuses 07's sea).
+
 ## Open questions (in priority order)
 
 1. ~~Which sea-chart approach (08–11) reads best, or which mix?~~ 11's navigation + 10's style (§9).