core: exclude dangling walls from room polygons

[wass08]

What does this PR do?

• Decomposes half-edge face walks into simple cycles before generating room polygons.
• Excludes dangling wall and graph-bridge excursions that previously introduced repeated vertices and self-intersections into auto-generated slabs and ceilings.
• Rejects incomplete face walks and adds a regression test for a closed room with a dangling interior wall.

How to test

1. Run bun test packages/core/src/lib/space-detection.test.ts.
2. Draw a closed four-wall room, then add a wall branch from one boundary vertex into the room.
3. Confirm the detected room, auto slab, and auto ceiling retain the original simple boundary without a triangular fill bleeding outside the room.

Screenshots / screen recording

N/A — the malformed private-project geometry was reproduced programmatically; the regression is covered by room-boundary assertions.

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

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Note

Medium Risk
Changes core room-graph extraction used by space detection, auto slabs/ceilings, and wallClosesRoom; logic is localized and regression-tested but affects all wall-driven geometry sync.

Overview
Room detection in extractRooms no longer treats half-edge face walks that trace out and back along dangling walls or graph bridges as a single room boundary.

After a walk closes, splitIntoSimpleCycles peels off simple cycles whenever a vertex repeats, and only those cycles become room polygons and boundaryFaces. Incomplete walks are dropped unless the traversal returns to the starting half-edge.

A regression test covers a closed four-wall room plus an interior branch from a corner: the detected space stays the original square (four walls, correct area) without pulling the branch into the boundary.

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