fix: process each component box as separate ManifoldSolidBrep in STEP export

[LiamConner10]

Summary

• Each component box from scene3d.boxes is now processed individually
• Creates a separate ManifoldSolidBrep for each component with its own ClosedShell
• Previously all triangles were merged into a single ClosedShell, violating STEP's requirement that each solid have its own watertight boundary

Root Cause

The original code collected ALL triangles from ALL component boxes into a single allTriangles[] array, then created ONE ClosedShell and ONE ManifoldSolidBrep from them. This violated STEP's constraint that each solid must have its own closed, watertight boundary - you can't merge multiple disconnected meshes into one shell.

The Fix

Process each box separately in the loop, creating individual solids:

for (const box of scene3d.boxes) {
  let boxTriangles: GLTFTriangle[] = []
  // ... get triangles for THIS box only
  if (boxTriangles.length > 0) {
    // Create INDIVIDUAL solid for this component
    const componentShell = repo.add(new ClosedShell("", componentFaces))
    const componentSolid = repo.add(new ManifoldSolidBrep(box.label || "Component", componentShell))
    solids.push(componentSolid)
  }
}

Test Plan

• All 10 existing tests pass (bun test)
• Type check passes (bunx tsc --noEmit)
• Snapshot updated for mesh-with-component01

Fixes #6

🤖 Generated with Claude Code

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