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fix(signals): merge clone-path folds onto containers privatized mid-batch (#3271)

A writable projection draft writing a descendant then an ancestor lost the
ancestor's writes: the descendant's fold path-copied through the ancestor
(privatizeCommitted re-pointed the parent slot at a fresh clone), so the
ancestor's clone-path fold swapped in its stale ensurePB-time backing and
failed the parent-slot CAS — orphaned, silently. Plain object stores fold
via the prototype-overlay flatten (in-place) and were immune; families and
arrays take the clone path.

drainFolds now detects the privatized container (t.v !== old) and merges
the batch's writes onto it in place: the trap's written-keys bound applies
exactly the written/deleted keys; an array length write (WK_ALL) falls back
to a value-diff against the pre-batch old, which leaves descendant-fold
slots untouched (same raw reference on both sides). Privatizes first so an
adopt-then-write batch never mutates an unowned adoptee.

+~90 B on the store scenarios; caps ratcheted with audit notes (measured
no-win golf this morning still stands).

Co-authored-by: Cursor <cursoragent@cursor.com>

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