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perf(signals): projection root writes take the overlay path (#3354)

Projection and derived-store drafts now open as prototype overlays like
plain stores (#3044), so a derive touching one root key of a wide keyed
record is O(written) instead of cloning the whole raw per recompute:
17.6 ms -> 0.02 ms per derive at 20k keys on the reporter's repro.
Optimistic families (view-seeding, truth-park), chained backings, arrays
and accessor containers keep the clone path. The overlay flatten is a
shared helper the write-override landing calls in place of the backing
swap.

Surfaced a pre-existing overlay bug: privatizeCommitted re-slotted the
parent unconditionally, so when the parent folded first in a drain and
the batch had replaced or deleted the child's slot, the child's flatten
resurrected it. Now a CAS like drainFolds' path copy. Regression tests
on plain stores in overlay.test.ts; deterministic clone-count tests and
a vitest bench pin the #3352 shape.

Size: createStore scenario 14.654 -> 14.696 KB, cap 14.75.

Co-authored-by: Claude via Cursor <noreply@cursor.com>

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