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fix(signals): preserve parked effects when transitions merge (#3310)

When a pending action's transaction merges into another (initTransition
called with a transaction while a different one is active), the outgoing
transaction's parked effects — its _queueStash, filled by every flush that
found it incomplete — were dropped on the floor. Signal values still
committed through the surviving transaction, but the render effects that
would have painted them never ran: stale DOM after a correct commit.

Restore the outgoing stash onto the live queue at the merge site. This
flush then parks those effects again under the surviving transaction or
runs them when it completes — the lifecycle every other parked effect has.
The outgoing transaction is dead after the merge (_done chased, removed
from `transitions`, never stashed into or completed), so its stash is not
reset; nothing reads it again.

Regression test from the PR: two actions, the second holding an optimistic
write the first later merges into; on `next` the render effect stays at
[0] after both settle, with the fix it reaches [0, 1].

No changeset: rc.7 is already versioned on `next` but unpublished (the
Release run was blocked by the Compiler-binaries harness bug fixed in the
previous commit), so the entry is folded directly into the rc.7 section of
packages/signals/CHANGELOG.md rather than triggering an rc.8 bump.

Closes #3310

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

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