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fix(signals): a held derivation is not a proposal — mainline writes become prev for the re-derivation (#3612) (#3630)

#2692's "manual write wins" precedence applies within a synchronous frame.
Across a hold, a held derivation is not a proposal: a mainline write to a
node whose held staging is a pass result (stamped by another transaction,
REACTIVE_MANUAL_WRITE not set on the node or its _firewall) becomes `prev`
for the transaction's re-derivation and does not suppress it. The write
still joins T per A34(1) — the writer's tick reveals with T, no intermediate
mainline commit. Writes made inside the transaction keep last-write-wins.
Applies to createSignal(fn) and createStore(fn) (CS-R31).

Mechanism: `setMemo` asks `heldDerivation` before the write, resolves an
updater against the committed value on a hit, takes the A34 join, and
`rederiveHeld`s (DIRTY + enqueue) instead of masking. The derived store
setter is wrapped by `derivedStoreWrite`, which records a held leaf whose
`_firewall` is the setter's node in `notifyWrites` and decides in its
`finally`. `updateIfNecessary`'s post-pull flag wipe now preserves
REACTIVE_MANUAL_WRITE: the mask is state, not scheduling, and a `latest()`
probe must not turn a proposal into a pass result.

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

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