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fix(signals): optimistic settle verdicts — combined isPending on the child lane, onSettled after the revert re-derive (#3409, #3411) (#3425)

optimistic value reported false while `isPending(a)` and `isPending(b)`
both reported true. The combined effect computed `true` but its run was
withheld: its two companion child lanes are siblings and merge, and the
parent lane's next notification then hit assignOrMergeLane's "subscriber's
lane was merged → take the source lane" shortcut, which skipped the
parent/child check and moved the effect onto the held parent lane, where
its verdict waited on the async it reports. A merged lane is now followed
to its root like any other and goes through the same check. -33 B on the
core floor.

revert. The settle drops the override in the commit pass but only
enqueues the reverted subscribers; the pass after re-derives them, and
reads do not pull. The one-shot fired in the commit pass's user phase saw
the source reverted beside a stale memo. The fire now waits for the heap
to drain (re-enqueues itself while `dirtyQueue` has work; `run` swaps the
queue so it lands in the next pass). Forcing the re-derive into the
commit pass instead breaks the lane frame of a write completing in its
own flush and the store clears (#3164 tear) — recorded in the internals
doc.

Pins for both (plus the no-action variant of #3411) in
optimistic-settle-verdicts.test.ts.

Co-authored-by: Claude <noreply@anthropic.com>

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