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fix(signals): a reader mounted mid-flight reads a derived store like its memo (#4491) (#4500)

* fix(signals): a reader mounted mid-flight reads a derived store like its memo (#4491)

`createStore(d => { d.v = m1() })` over an async memo whose input reads
`latest()` has a derive that is a verdict reader through what it reads
(D-25 / F-6). At a re-ask it is served `m1`'s committed value and settles
over it, so its leaves read as final: a reader that mounted mid-flight
showed the old question's answer before the new one (in the mounting
flush once #4479 skips the unread derive, at the landing before it),
where a reader of `m1()` is held (F-2). D-30: a derived store reads like
its memo.

`pullFamily`: a tracked reader of a settled verdict-reader derive with no
window of its own reads the flights among the derive's deps, as the
memo's reader would, so it is held; the derive is that hold's work until
it answers. `runProjectionComputed`: a derive whose pending is on its
leaves' readers (`fam.woke`) reads plainly, as a pending node's pass does
(#3884), so its landing is the answer the held reader shows. Store-only:
no frozen floor moves.

The #4479 review's probe forms (key read, Object.keys, JSON) are pinned
against the memo form: mounted mid-flight with and without a plain write
beside the flight, on screen at the re-ask, and mounted after the landing.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015hPxMkC3jg3Uivh65gvaf2

* fix(signals): gate #4491 on the verdict engine; the derive's posture is never written outside its pass

Review of a78d91a32: a verdict reader exists only with the verdict engine,
so both halves of the fix now sit behind `observeFlightHook`. Store bundles
without isPending/latest fold them away (createStore, compiled CSR and
compiled hydrating at +0 B vs next), so the cap raises recorded for
createStore and compiled CSR are dropped.

The pull no longer clears the derive's CONFIG_... (continued)

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