refactor(capture): extract assembly validation

[w1ne]

Summary

• Extract assembly-specific session validation and solved-joint normalization into src/modeling/capture/assemblyCaptureValidation.ts
• Keep CaptureSession as the owner of register/createShape, record IDs, and metadata.paramRefs
• Add direct validation/orchestration tests for exact errors, malformed-joint behavior, no record append on failed validation, and paramRefs preservation

Verification

• npm test -- tests/unit/capture/assemblyCaptureValidation.test.ts 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 tests/unit/kernel/architectureBoundary.test.ts tests/integration/lowering/sceneToCompoundUnion.test.ts tests/integration/lowering/solvedAssemblyReactive.test.ts
• npm run typecheck
• npx eslint src/modeling/capture/captureSession.ts src/modeling/capture/assemblyFeatureRecords.ts src/modeling/capture/assemblyCaptureValidation.ts tests/unit/capture/assemblyCaptureValidation.test.ts tests/unit/capture/assemblyFeatureRecords.test.ts
• npx depcruise --config .dependency-cruiser.cjs --output-type json src/modeling/capture/assemblyCaptureValidation.ts src/modeling/capture/assemblyFeatureRecords.ts

Review

• Architecture reviewer: no blockers
• Behavior reviewer: no blockers

No docs or generated assets changed.