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fix(signals): a derived store heals with its Errored like the memo it derives from (#4451) (#4522)

A derive's lane-pass write to a key nobody had read yet stages on the
container (`stageOn` → `holdWithDerive` lists it in the lane, #4478); the
leaf born from that staging in the same flush was a plain staging the park
then held with the frame, so frame readers saw the committed `undefined`
beneath while the memo the derive reads showed the lane's value. An
`Errored` over the store stayed on its fallback until the action landed; a
memo over the key derived NaN.

`bornStaged` now hands a leaf born from a lane container's staging (OVERRIDE
without GUESS) to `holdWithDerive` with the container's lane: the staging
becomes the lane's derivation, as the write would have left a leaf that
existed then (E3, D-25, D-39). `holdWithDerive`'s `userWrite` discriminator
moves to its callers so the born leaf — a read, possibly inside a user
setter's body — is classified by the container's staging, not the caller's.

The #4137 pin's mid-action screen changes from `["a",null]` to `["a","b"]`
(accepted by the maintainer): the derive's new key is the lane's derivation
for a leaf born after the write as for one that existed; the memo twin
shows `["a","b"]` on both.

Size: "app: hydrating + every store primitive family" 32.48 -> 32.50 KB
(+48 B minified / +58 B brotli over next on that scenario; core floor,
hello world and every bundle without stores 0 B). Accepted by the
maintainer.

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

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