refactor(candidate): share canonical serialization

[drewstone]

Candidate identity passed through two serializers, then compared their hashes. This change uses Interface's canonical bytes for candidate identity and builds each frozen document from its captured bytes. It removes the duplicate encoder and serializer-disagreement branch. Native SHA-256 remains in place for workspace archives.

Valid persisted digests remain unchanged. Invalid JSON is refused consistently, and changing input getters cannot separate the returned value from its bytes. Public exports are unchanged.

Validation:

• Baseline candidate suites: 75 passed. Updated core suite: 14 passed.
• Lint, source/examples typecheck, build, packed-package verification, edge Worker execution, and generated-document freshness passed.
• Independent review confirmed 17 valid JSON compatibility cases and the minimum Interface peer version.
• Full local suite with four workers: 3,507 passed, 9 skipped. The initial run hit two fixture deadlines under concurrent repository checks; both also passed in isolation. Hosted CI must pass before merge.