Complete forward-delete and batched-transaction compatibility coverage

[haoranyu]

Summary

• add an explicit real-CM6 delete.forward regression that locks the legacy d wire shape and v2 playback state
• pass the exact packaged CM6 forward-delete payload through both the published CM5 and packaged CM6 players at every natural operation boundary
• dispatch two sequential CM6 transactions in one real ViewUpdate, then replay the resulting payload through both players with explicit timestamp, document, and selection oracles

Refs #49.

Why

The final #49 audit found two narrow coverage gaps: forward deletion was only implied by the generic delete.* implementation, and a genuine multi-transaction ViewUpdate was only checked at serialization time. These tests make both behaviors part of the release compatibility gate without changing runtime code or package contents.

Verification

• npm run test:all
• npm run test:coverage — 126/126 tests; 76.84% statements, 70.33% branches, 90% functions, 76.75% lines
• npm run test:compat after the final forward-delete addition
• exact published CM5 1.1.5 and maintained 1.1.8 compatibility checks
• npm audit --audit-level=moderate — 0 vulnerabilities
• npm pack --dry-run --json — 31 expected package files
• independent review — no findings