fix(markdown): persist browser editor changes

[George Ng (GeorgeNgMsft)]

Summary

Final layer of the replacement stack for #2940, based on #2969 (georgengmsft-markdown-browser-server).

• persist browser autosave and explicit saves with binding tokens and expected revisions
• preserve serialized Markdown syntax and surface serializer failures
• reject stale or incomplete binding/snapshot/switch data and reconcile only identical-content conflicts
• preserve same-file bindings, nested document paths, and opaque collaboration room IDs
• assign and transfer the primary browser autosave role while retaining headless compatibility

Validation

• markdown package build
• focused browser persistence and view-service suites: 17 tests passed
• full markdown suite: 8 suites, 64 tests passed
• package Prettier check
• lint, complexity, circular-dependency, and test-debt ratchets
• git diff --cached --check

The layer changes 1,377 lines, below the 1,500-line cap.