fix(markdown): apply document edits deterministically

[George Ng (GeorgeNgMsft)]

Summary

Layer 2 of the replacement stack for #2940, inserted to keep the stack reviewable.

• apply Markdown operations against stable base-relative character offsets
• preserve deterministic source order for inserts at the same position
• reject overlapping and out-of-range operations
• serialize Markdown content nodes and support adding/removing strong, emphasis, code, and link marks
• correctly remove link marks with balanced parentheses only when the destination matches

Deferred to higher layers

Durable workspace persistence, revision and binding conflict checks, streaming persistence, view-service state, browser synchronization/autosave, same-file editor rebinding, and loopback-only binding remain intentionally deferred.

Validation

• cached TypeScript 5.4.5 strict no-emit check: passed for markdownOperationSchema.ts and documentOperations.ts
• direct compiled smoke tests: passed for base-relative edits, same-position inserts, overlap rejection, mark add/remove, balanced-parenthesis links, and mismatched-link preservation
• cached Prettier 3.5.3 changed-file check: passed
• git diff --check: passed
• focused review: fixed link removal corruption for parenthesized or mismatched destinations

The normal package build/Jest suite and PR ratchets could not run because this worktree had no node_modules; the single time-boxed cached/offline restore did not complete and attempted unavailable Azure feed artifacts. The commit hook was therefore bypassed after the equivalent cached Prettier/type/smoke checks above.