perf: make guarded proposed changes atomic and fast

[developer-1px]

Summary

• reuse one private copy-on-write sequential-replace seam for overlapping patch application and history inverse capture
• keep leading RFC 6902 guard tests on the same fast path as their mutation suffix
• accept proposed changes as one atomic guard-test + proposed-operation patch without changing the public API
• reserve an accepting proposal through document publication so subscriber reentrancy cannot remove, reject, or replace it
• preserve stable error classification, history/undo/redo semantics, and merged generated-id behavior during reentrant loads

Performance

50k-item document, 1,000 overlapping replace operations:

• inverse: 41.79ms → 0.71ms (~59x)
• history=100 patch: 43.18ms → 1.21ms (~36x)
• guarded history=0 patch: 46.32ms → 0.70ms (~66x)
• guarded history=100 patch: 92.14ms → 1.41ms (~65x)

Validation

• npm run verify -w @interactive-os/json-document (457 passed, 4 skipped)
• npm run verify -w @interactive-os/json-document-proposed-changes (21 passed)
• npm run extensions:verify
• npm run standard:check
• npm run docs:check
• git diff --check