Computes Matrix4x4 per Setup part by walking PlacementFrames[Resting] →
[Default] → first-available, matching SetupMesh.Flatten's priority.
Foundation for #56 fix: ParticleHookSink will use these to apply each
CreateParticleHook's PartIndex-relative offset to the right mesh part.
4 xUnit tests cover Resting-over-Default preference, Default fallback,
empty-PlacementFrames returns empty, DefaultScale application.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
