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

Historical validation before the September 5, 2026 rebase (not rerun in this worktree):

• 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

Rebase validation: the single commit is patch-equivalent to the original (git range-diff reports =); all 11 non-lockfile changed files are byte-identical, and the lockfile retains the same three-line dependency addition. Upstream session/workspace compatibility files are unchanged. Ancestry and git diff --check pass. Build, tests, formatting, and ratchets were not rerun because this worktree has no .npmrc, node_modules, or compiled test output; dependencies were not provisioned for the restack.

The layer still changes 1,377 lines (12 files, +987/-390), below the 1,500-line cap.