refactor(capture): extract assembly record builders

[w1ne]

Summary

• Extract assembly feature record construction and solved-pose validation from CaptureSession into src/modeling/capture/assemblyFeatureRecords.ts
• Keep CaptureSession responsible for session membership/kind validation and shape registration only
• Add byte-stability, regression, and architecture-boundary coverage for assembly record builders

Verification

• npm test -- tests/unit/capture/assemblyFeatureRecords.test.ts tests/unit/assemblies/assemblyCapture.test.ts tests/unit/assemblies/subAssemblyComposition.test.ts src/modeling/mates/validator.test.ts tests/integration/lowering/sceneAssemblyModel.test.ts tests/integration/lowering/coloredPosedAssembly.test.ts src/modeling/capture/posesGate.test.ts src/modeling/mates/poseEnvelope.test.ts src/modeling/mates/workspaceReachability.test.ts tests/integration/lowering/sceneBackendEmission.test.ts tests/integration/lowering/sceneMultiMesh.test.ts tests/unit/kernel/architectureBoundary.test.ts
• npm run typecheck
• npx eslint src/modeling/capture/captureSession.ts src/modeling/capture/assemblyFeatureRecords.ts tests/unit/capture/assemblyFeatureRecords.test.ts
• npx depcruise --config .dependency-cruiser.cjs --output-type json src/modeling/capture/assemblyFeatureRecords.ts

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