Replace OT cursor tracking with Automerge cursors in presence (#129)

* test: cover OT cursor tracking race cases before refactor

Adds usePresence.test.ts with five regression tests covering PR #94
(cursor tracking across concurrent local edits) and PR #110 (end-of-line
cross-line flash guard). Tests are written at the decoration-position
level so they survive the Automerge cursor refactor tracked in #113.

* refactor(presence): replace OT offset tracking with Automerge cursors (#113)

Remote cursor/selection travel as Automerge cursor strings anchored to
['text'] and resolve on the receiver against its local doc. Deletes the
OT machinery (transformOffset, peerStateRef, anticipatingEditRef,
same-line guard) and the onDidChangeContent OT effect; modelVersion
stays as a re-render trigger.

`move: 'before'` for selection end is a no-op in automerge 2.2.9 — using
default 'after' everywhere; deviation recorded in the plan.

* test(presence): lock in sender-side cursor ordering

* test: pin automerge cursor semantics as probe fixture

Author: shikokuchuo