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fix(signals): a first answer landing in the tick a write re-asks it is answered, then re-asked (D-22, F-9) (#4459)

* fix(signals): a first answer landing in the tick a write re-asks it is answered, then re-asked (D-22, F-9)

An async memo's first answer resolves, and in the same synchronous turn a
plain write re-asks it; the promise reaction runs after the write, so the
landing and the re-ask are processed in one flush. The runtime treated the
answer as never produced:

- `asyncWrite` dropped it outright when the write's notification had marked
  the node dirty (a stale-answer guard meant for re-asks).
- Otherwise it landed staged, the re-ask left the node pending and
  uninitialized, and `isPending` gave it no verdict (`false` from the
  flush's probe, never re-run), while an identity memo or a derived store
  over it never derived from it (`latest` of either threw: [ROOT-L]).

Now a first answer always lands; `isPending` of a node answered and
re-asked in one flush is the re-ask's verdict (true); and a memo with no
value of its own that reads it derives from the answer once, then re-runs
on the missed-wake latch and waits on the re-ask holding it (a verdict
engine hook, so the hello-world floor pays only the call). Amends D-22's
2026-10-28 clause per the 2026-10-30 ruling: a plain write re-asking the
memo carries the answer like an action's.

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

* fix(signals): a first answer landing in the tick a write re-asks it is answered, then re-asked (D-22, F-9)

An async memo's first answer resolves, and in the same synchronous turn a
plain write re-asks it; the promise reaction runs after the write, so the
landing and the re-ask are processed in one flush. The runtime treated the
answer as never produced:

- `asyncWrite` dropped it outright when the write's notification had marked
  the node dirty (a stale-answer guard meant for re-... (continued)

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