refactor(offline): simplify repository mutation paths

[rdlabo]

Summary

• unify offline state mutations around atomic transactReplica while retaining deprecated compatibility aliases
• remove redundant enqueue normalization, canonical JSON construction, and duplicate replica reads
• trim obsolete test scaffolding and add payload-validator and compatibility regression coverage

Compatibility

• preserves the optional custom-repository read contract and tombstone capability guard
• leaves the durable schema and legacy migration fields unchanged
• requires no Seatkeep or Winecode consumer changes

Validation

• npm run lint
• npm test: 74 files, 1,127 tests
• offline suite: 19 files, 633 tests
• SQLite concurrency: 2 tests
• real SQLite contract: 8 tests
• npm run test:offline-types:nonstrict
• npm run test:package-consumer
• npm run prebuild:kit
• git diff --check
• manager review: APPROVE
• third-party acceptance review: ACCEPT

Dependency

Stacked on #99 so this PR contains only the offline simplification commit.