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fix(signals): a memo over a re-asked async memo is born held with the mount that first reads it (#4504) (#4509)

* fix(signals): a memo over a re-asked async memo is born held with the mount that first reads it (#4504; D-2, D-22, D-25, D-27)

An async memo `m1` reads `m0 = createMemo(() => latest(s) + s())`; a write
re-asks `m1` while nothing reads it, and mid-flight a render effect mounts
reading `mb = createMemo(() => m1())` (or `m1() * 10`). The screen was
`--mounted, r:2, r:4`: the held reader published the old answer at the
landing and the re-ask's only at the commit. Reading `m1` directly, or with
a plain input, gives `--mounted, r:4`.

Cause (core.ts `read`, the #4050 posture inheritance): `mb`'s pass in the
mount was not a verdict reader (it was pending, so `recompute`'s head
cleared CONFIG_VERDICT, the #3884 clause of D-25) and its plain read of the
in-flight `m1` threw, holding the mount (D-2, F-2). But the read stamped
`m1`'s CONFIG_VERDICT onto the pass before throwing, so `mb`'s next run —
the hold's re-run at the landing — was a verdict reader and was served
`m1`'s committed value (`stagedScreen`), and the effect published it.

Fix: a read of a pending node hands on no posture to a pass that is not a
verdict reader already. Such a read throws: it read no value, and a node
whose flight is up is the hold's work, not a verdict reader (D-25, #3884);
a memo's posture is what it reads. A verdict reader served the screen of a
flight (`observeFlight`) keeps the posture, so a memo already on screen
before the re-ask shows exactly what it showed before. The plain mount is
not display-ahead (D-27): its reader is born held and shows the re-ask's
answer (D-22's identity-memo clause, F-9); `latest`/`isPending` of `mb`
read `20`/`true` during the flight (D-22, D-23 "a derivation of it ... at
any depth").

The line sits behind `observeFlightHook`, which folds away in bundles
without the verdict engine (core floor, hello world: 0 B); +15 B minified
where... (continued)

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