refactor(capture): extract authoring record builders

[w1ne]

Summary

• move dfmSpec, curve3d, embossText, and projectCurve record construction out of CaptureSession
• keep face selector resolution in CaptureSession while authoringFeatureRecords stays a record-builder leaf
• add byte-stable capture record coverage and helper boundary assertions

Verification

• npm test -- tests/unit/capture/authoringFeatureRecords.test.ts tests/unit/capture/dfmSpec.test.ts tests/unit/capture/nurbsCurve.test.ts tests/unit/capture/embossText.test.ts tests/unit/capture/projectCurve.test.ts tests/unit/capture/shapeEmbossProjectProxy.test.ts
• npm run typecheck
• npx eslint src/modeling/capture/captureSession.ts src/modeling/capture/authoringFeatureRecords.ts tests/unit/capture/authoringFeatureRecords.test.ts
• npm test -- tests/unit/kernel/architectureBoundary.test.ts
• git diff --check

Subagent review: behavior-loss review found no issues; architecture review found one helper/proxy type dependency, fixed and re-reviewed clean.