core: shared pure plan-footprint math for spatial-grid and MCP

[ActArtech]

What does this PR do?

Extracts rotation-aware plan (XZ) footprint math into a pure, Node-safe core module so spatial-grid, alignment anchors, and (later) MCP layout clearance share one formula after #569.

• Add packages/core/src/lib/plan-footprint.ts: corners, AABB, expand-then-intersect gap overlap, scaled item helper (null for wall/ceiling hosts)
• Spatial-grid manager and alignment-anchors delegate to it (no third parallel AABB path)
• Export @pascal-app/core/plan-footprint (pure) and re-export from @pascal-app/core/spatial-grid
• Design note: wiki/architecture/plan-footprint.md (one source; packing gap ~0.08 vs check/verify gap: 0)
• Unit tests for rotation, scale, gap semantics, corner/AABB parity

Out of scope: rewiring MCP furnish_room / verify_scene / door keep-outs to call these helpers (follow-up).

How to test

1. bun test packages/core/src/lib/plan-footprint.test.ts — expect all pass
2. bun test packages/core/src/services/alignment-anchors.test.ts — still green
3. bun run --cwd packages/core build — tsc --build clean
4. Optional: bun run check packages/core/src/lib/plan-footprint.ts packages/core/src/lib/plan-footprint.test.ts packages/core/src/hooks/spatial-grid/spatial-grid-manager.ts packages/core/src/services/alignment-anchors.ts
5. Optional smoke: import from @pascal-app/core/plan-footprint and @pascal-app/core/spatial-grid and confirm planFootprintAABB returns the same box

Screenshots / screen recording

N/A — non-visual change (pure geometry helpers + tests + wiki note).

Checklist

• I've tested this locally with bun test / bun run --cwd packages/core build
• My code follows the existing code style (bun run check on changed files)
• I've updated relevant documentation (wiki/architecture/plan-footprint.md)
• This PR targets the main branch

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Note

Medium Risk
Unifies alignment and floor-collision footprint math (intentionally); wrong rotation convention would affect placement, snapping, and future MCP clearance, though tests and delegation to prior spatial-grid formulas mitigate that.

Overview
Introduces packages/core/src/lib/plan-footprint.ts as the single pure XZ source for rotation-aware footprint corners, conservative AABBs, expand-then-intersect overlap with configurable gap, and a scaled planFootprintAABBForItem helper (null for wall/ceiling hosts).

Spatial-grid and alignment-anchors stop inlining duplicate math and delegate to this module; the spatial-grid entry also re-exports the helpers. A new @pascal-app/core/plan-footprint package export targets Node/MCP callers without pulling the grid manager.

Adds unit tests for rotation, corner/AABB parity, gap semantics, and item scaling; documents the module in wiki/architecture/plan-footprint.md. MCP furnish/verify rewires are explicitly out of scope.

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