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 and alignment anchors share one formula (follow-up seam for MCP layout clearance after #569).

• Add packages/core/src/lib/plan-footprint.ts with planFootprintCorners and planFootprintAABB only (plus PlanAabb / PlanVec2 types)
• Spatial-grid manager and alignment-anchors delegate to it (no third parallel AABB path)
• Export @pascal-app/core/plan-footprint (pure) and re-export the same two helpers from @pascal-app/core/spatial-grid
• Gap call-site meanings (gap ≈ 0.08 packing vs gap = 0 hard verify) documented in the module header for the MCP consolidation follow-up
• Unit tests for unrotated / 90° / 45° AABB and corner↔AABB parity

Narrow response to review: dropped unused aabbsOverlapPlan, planFootprintAABBForItem, and planFootprintAABBFromCorners (no dead / speculative exports).

Out of scope (follow-up): rewiring MCP door-clearance / layout-clearance / check-collisions onto these helpers and deleting MCP's duplicate overlap path.

How to test

1. bun test packages/core/src/lib/plan-footprint.test.ts — all pass
2. bun test packages/core/src/services/alignment-anchors.test.ts — still green
3. 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 — clean
4. Optional: import planFootprintAABB from @pascal-app/core/plan-footprint and from @pascal-app/core/spatial-grid and confirm the same box

Screenshots / screen recording

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

Checklist

• I've tested this locally with bun dev
• My code follows the existing code style (run bun check to verify)
• I've updated relevant documentation (if applicable)
• This PR targets the main branch

Note: this change is pure math with no UI surface; verification was bun test on plan-footprint + alignment-anchors, bun check on touched files, and green GitHub CI (quality / mcp-ci / cli-smoke).

[Aymericr]

Thanks for picking up the note I left at the end of #569 — this is the right direction and I checked the math rather than assuming: planFootprintAABB's abs-cos/abs-sin extents are provably the same box as the rotated-corner loop it replaces in alignment-anchors.ts, planFootprintCorners preserves spatial-grid's rotation convention exactly (different corner order, nothing depends on order), the attachTo null-list matches the enum in item.ts, and it stays clear of the R3F guard in architecture.test.ts. The alignment-anchors.ts comment admitting it "mirrors the rotated-corner math the spatial-grid manager uses" was a real duplication and −58 lines is a real removal.

But as it stands the PR moves the number in the wrong direction, and that's what I want to fix before it lands.

The ask on #569 was that MCP stop being a third copy. With the rewire declared out of scope, aabbsOverlap in door-clearance.ts survives — and the new aabbsOverlapPlan is a line-for-line reimplementation of it. Post-merge there are three copies of expand-then-intersect instead of two, and MCP still uses its own. The extraction is the easy half; the consolidation was the point.

Three of the five exports have no callers. aabbsOverlapPlan and planFootprintAABBForItem exist for the deferred MCP work; planFootprintAABBFromCorners is called only by tests, which then assert it's identical to planFootprintAABB — so it's an identity helper proving itself. On #569 I asked you to delete aabbFromPlan and nodeItemAabb for exactly this, and AGENTS.md names dead code and speculative abstractions specifically.

Two paths, either fine by me:

• Finish it. Keep all five exports and rewire door-clearance.ts / layout-clearance.ts / check-collisions.ts to import from @pascal-app/core/plan-footprint, deleting MCP's aabbsOverlap and PlanAabb. Heads-up on the trap that probably made you defer: @pascal-app/core is only a devDependency of packages/mcp while door-clearance.ts already runtime-imports getScaledDimensions from @pascal-app/core/schema — that's a latent packaging bug, and moving more runtime code across that edge means promoting it to a real dependency. Please also keep the gap: 0 versus 0.08 distinction from #569 intact per call site; that's the bug I'd most want a test to hold.
• Or narrow it. Ship only planFootprintCorners and planFootprintAABB, drop the other three exports and their tests, and leave the subpath export in as the seam for the follow-up.

On the wiki page: I'd rather it didn't say "Aymericr noted…" — the other wiki/architecture/ pages state the rule, not who asked for it, and 43 lines of page restating the header of a 133-line math module is more provenance than durable rule. Fold the gap-semantics table into the module header and skip the page, or cut it to the invariant plus the call-site table.

Useful side-finding worth carrying into the rewire: your aabbsOverlapPlan comment ("expand each box by gap") is right and MCP's existing one ("expand each box by gap/2") describes something the code doesn't do. Fix that comment while you're in there.

[ActArtech]

Took the Narrow path from the review:

• Public surface is now only planFootprintCorners + planFootprintAABB (plus PlanAabb / PlanVec2 types).
• Dropped aabbsOverlapPlan, planFootprintAABBForItem, and planFootprintAABBFromCorners with their tests.
• Removed wiki/architecture/plan-footprint.md (no named attribution). Gap call-site meanings (0 vs ~0.08) live in the plan-footprint module header as the seam for the MCP consolidation follow-up.
• @pascal-app/core/plan-footprint subpath export kept.

Follow-up (not in this PR): rewire MCP door-clearance / layout-clearance / check-collisions onto these helpers and delete MCP's duplicate overlap path.