CircuitJSON Toolkit 1.4.2

CircuitJSON Toolkit 1.4.2 removes repeated immutable-graph work from
toolkit-owned document rebuilds and cooperative structured-clone adoption.

Changes:

• Reuse already branded, frozen extension roots when rebuilding validated
  documents.
• Validate each acquired clone container and freeze it atomically rather than
  redefining every property.
• Preserve defensive handling for arbitrary caller data and cancellation
  checkpoints for cooperative work.
• Add packaged release notes and ownership/atomicity regression coverage.

Verification:

• Complete multi-phase test suite: 12 + 520 + 18 + 1 + 11 + 5 + 10 tests,
  zero failures.
• npm run check:format
• npm run check:features (1,207 mappings)
• npm publish --dry-run (circuitjson-toolkit-1.4.2.tgz, 194 entries,
  424,001 bytes packed)