[laneA] M1: Stage viewport, retarget port, dock+inspector
- room3d.js: pure-module port of rigroom retarget, target parameterized;
_selftest bakes across two mismatched-rest skeletons (maxErr 6.4e-12)
- stage.js: Stage class (IBL, gizmo, entities, clip bake cache) per PLAN §4.2
- dock.js: asset dock + inspector, offline file-drop + clip retarget
- index.html/style.css: grid layout, guarded timeline import (page survives B absent)
Verified in-browser end-to-end: Mixamo char loaded+normalized, backdrop plane,
gizmo move, running clip retargeted (66 tracks) and animating live.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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