docs(plan): indoor walkable-plane BSP port — implementation plan
Five tasks (TDD throughout):
1. Extend FindWalkableInternal signature with ref ushort hitPolyId
(mechanical, two existing callers pass discards).
2. Add BSPQuery.FindWalkableSphere wrapper + 4 unit tests
(two-floors-foot-between, only-upper-floor-foot-above, no-walkable-
in-range, steep-poly-rejected).
3. Refactor TryFindIndoorWalkablePlane through BSPQuery.FindWalkableSphere
with sphereRadius thread + WalkableAllowance save/restore, delete
PointInPolygonXY, update single callsite, integration test for
two-overlapping-floors + allowance-preservation.
4. Add [indoor-walkable] probe line on existing PhysicsDiagnostics flag.
5. Visual verification by user + roadmap + ISSUES #83 close.
Spec: docs/superpowers/specs/2026-05-19-indoor-walkable-plane-bsp-port-design.md
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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