Preserve Roughdraft source and preview through plain-text edits

Docs intentionally dispatches client-generated review edits through the ordinary collaborative text protocol. Two clients starting from the same file without endmatter can each add a valid first comment. Rebasing preserves both insertions but creates two YAML footers; Roughdraft then reports a missing metadata entry. This limitation is explicitly accepted for the initial implementation.

A related stale-client case is pinned as STALE ENDMATTER APPEND: one client creates its first comment and footer while another appends at the old EOF. Text OT preserves that stale append after the new footer, where it corrupts YAML. It cannot be fixed by relocating remote changes in one client, which would diverge from the shared document.

The executable reproduction is apps/os/src/domains/workspaces/collab-review.test.ts, named “simultaneous first comments should share one valid endmatter”. It uses the real Roughdraft writer, client text diff, collaboration host and parser, wrapped with createFailing. Only the duplicate-footer failure is expected; unrelated errors or an unexpected pass fail the suite.

Exit criteria: both comments and the original prose survive, the resulting file has one valid endmatter, and the test runs without the expected-failure wrapper. Keep the solution small; do not introduce a separate comment store or collaboration engine.

The same test file now pins concurrent word/paragraph comments in both arrival orders, with endmatter already present. Both local edits are valid and all three comments survive, but the merged nested highlight leaks {== / ==} into the preview without a parser diagnostic. Only that exact CONCURRENT COMMENT OVERLAP failure is expected. A fix must preserve the original prose and both annotations without adding a separate annotation protocol; remove the expected-failure wrapper once the preview stays correct in both arrival orders.

Two upstream source-preservation limitations are pinned in the same file:

• UNRELATED COMMENT REWRITE: adding a comment reflows an existing long body and removes timestamp quotes. The comment still reads correctly, but the text diff touches another author's entry. A fix must keep unrelated entries byte-for-byte intact, so concurrent edits and Git/redline attribution remain local.
• ORPHANED COMMENT ENDMATTER: ordinary typing removes an inline comment while leaving its metadata. Roughdraft stops recognizing that footer, renders it as prose, and appends a second footer for the next comment. A fix must recognize the remaining metadata and preserve a single endmatter with a clean preview.

Keep these as expected failures until there is a small upstream or adapter fix; do not build another serializer, annotation format, or repair protocol around them.