fix(ifc): avoid merging adjacent wall assemblies

[yorhodes]

Summary

• tighten the wall-fragment centerline tolerance so adjacent parallel walls are not collapsed
• require material associations to match before collinear fragments merge
• treat a tagged wall and an untagged wall as incompatible instead of silently allowing the merge
• add focused regression coverage for offset centerlines and material boundaries

Scope

This is the cleanup-only follow-up requested in the review of #553. It changes only the IFC cleanup implementation and its tests.

Fixture wall counts

The same unsimplified converted nodes were passed through main and this branch:

Fixture	Input walls	main output	this PR output
01-duplex.ifc	57	53	57
04-ifc-open-house.ifc	4	4	4
05-paris-ground-floor.ifc	114	85	100
10-sample-house.ifc	5	5	5

For Paris, merged groups change from 15 to 8 and removed merged walls from 28 to 13. The stricter behavior intentionally preserves ambiguous wall assemblies for later inspection instead of joining nearby centerlines.

Verification

• bun test packages/ifc-converter/tests/cleanup.test.ts — 6 passed
• Biome check on both changed files
• before/after conversion counts recorded for all four bundled fixtures

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Note

Medium Risk
Changes post-conversion scene topology (fewer merged walls on real fixtures like Paris), which can affect downstream geometry and editing but is confined to the IFC simplification step with explicit regression tests.

Overview
IFC scene-graph cleanup now merges collinear wall fragments less aggressively, so adjacent parallel assemblies and mismatched materials stay separate.

Centerline tolerance in wallLineTolerance drops from ~half wall thickness to ~10% (capped ~5–25 mm), so fragments offset by a couple of inches no longer collapse into one wall.

Material gating adds wallMaterialSignature / wallMaterialCompatible on metadata.material and metadata.materialLayers; tagged vs untagged walls are incompatible, while identical layer assemblies can still merge. wallsCanMerge enforces this before interval checks.

Regression tests cover offset centerlines, material/layer boundaries, and matching-layer merges.

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