refactor(workspace): unify YKeyValueLww reconciliation

[braden-w]

YKeyValueLww had two conflict resolvers: constructor hydration and a roughly 180-line observer callback. They implemented the same timestamp and positional policy through different branch trees, while overlay cleanup depended on Yjs net-delta entries that do not include add-then-delete operations.

This makes reconciliation one owned transition:

constructor snapshot ─┐
                      ├─> reconcile affected keys
Y.Array event facts ──┘     -> select one winner per key
                             -> update the confirmed index
                             -> delete visible losers
                             -> emit one original-origin batch

selectWinner is the single policy boundary: higher timestamp wins, with final Y.Array position deciding equal timestamps. The transaction-local read state is one discriminated, transaction-owned overlay rather than independent write and delete collections. Constructor hydration and live events preserve the same stored entry objects, so unchanged values keep the reference identity required by ADR-0077.

The no-conflict path stays delta-sized. Array snapshots and entry-position maps remain lazy, so unique bulk inserts avoid an O(total rows) scan while bulk conflicts still resolve from one snapshot. Single-key writes keep eager cleanup; a final delete now removes every visible version for the key, preventing repeated unconfirmed writes from resurrecting it.

Persisted { key, val, ts } data, the published class and ObservableKvStore surfaces, original transaction origins, cleanup transactions, observer batching, stable references, and existing event actions remain unchanged.

One existing boundary remains deliberately unresolved: a public change handler that writes while conflict cleanup is queued joins the private cleanup transaction in Yjs. Defining whether that write should emit a second public batch, and under which origin, is an event-contract decision outside this internal ownership rewrite.

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