codeSplitter line locks 3: word-level rebase of stale line edits #928

Design: apps/codesplitter/docs/line-locks.md (#925).

On a stale set, read the base text from op:<base> and try mergeWords(base, theirs, mine) from lines.ts. If the words don't overlap, land it (blamed on the sender); otherwise answer the 409 as today. This is the narrow, code-safe version of in-line CRDT/OT.