fix(markdown): secure browser document bindings

[George Ng (GeorgeNgMsft)]

Summary

Layer 4 of native stack #2970, rebased in place onto current main after #2968 merged.

• validate browser and IPC reads/writes against the active binding token, workspace identity, content revision, and a single-use durable-read provenance token
• preserve nested relative paths and isolate same-basename documents with binding-scoped collaboration rooms
• serialize browser binding transitions and SSE processing so stale switch responses cannot attach the editor to the wrong room
• reject stale/concurrent writes, detect external disk edits throughout browser-backed reads, and prevent AI snapshots from overwriting newer editor changes
• hydrate the authoritative Yjs document on file load and keep HTTP content/revision snapshots coherent
• keep the Markdown HTTP/WebSocket service on IPv4 loopback and advertise its bound loopback port
• extract focused IPC and SSE handlers without changing protocol behavior so the complexity ratchet remains at baseline

Rebase

• Replayed only fix(markdown): secure browser document bindings #2969's two commits from historical parent ab068c61a3275cad6762f82d0a10de6338e37931 onto main at c9eeb933b317afe2e29a432b325eabf02e8c1871.
• Preserved current main's existing-only /file/load behavior and adapted browser fallbacks to the hardened asynchronous bound-document reader.
• Current head: 256e472ae4081646a95376dda6ffce7089ab6f4a.
• Diff against main: 14 files changed, 2,175 insertions, 622 deletions.

Validation

• pnpm run build markdown
• pnpm --filter '@typeagent/markdown-agent' test — 98/98 tests passing
• Prettier on all changed files
• lint ratchet against origin/main
• complexity ratchet against origin/main — over-threshold counts remain unchanged at 1 cyclomatic and 1 cognitive
• circular-dependency ratchet against origin/main
• debt-marker gate against origin/main
• git diff --check
• repeated adversarial reviews with GPT-5.6 Sol and Grok 4.6; findings were fixed and regression-covered
• final complexity-only refactor independently reviewed by GPT-5.6 Sol and Grok 4.6 with no substantive findings

Stack

Native stack #2970 remains unchanged and ordered as [2966, 2967, 2968, 2969, 2971], including merged #2966, #2967, and #2968. This PR remains draft with base main; no stack metadata or #2971 branch content was modified.