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fix(signals): latest() opens no transaction for its verdict; its reader watches the seam (#4365) (#4393)

* fix(signals): latest() of a memo born in a lane throws NotReady instead of undefined (#4362, #4363, #4376)

A memo whose compute reads latest(s) is re-asked as the write's verdict
lane work, so it becomes a lane derivation. While its first pass is in
flight, nothing is in the lane, and laneValueOf/display fell back to the
committed value, which an uninitialized node does not have:

- latest(m)/isPending(m) served undefined and probed it pending (#4363);
- the lane's end committed that undefined as the node's answer and
  cleared STATUS_UNINITIALIZED with the flight still up, which tripped
  the flush loop guard for [latest(m), isPending(m)] (#4362);
- in hydration, m2 = fetchC(1 + latest(m1)) asked fetchC(NaN) instead of
  adopting the streamed answer (#4376).

laneValueOf/display now return NOT_PENDING for such a node, so every
read throws NotReady (A7; A19 exc. 1) and the lane's end leaves the node
in flight.

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

* fix(signals): latest(a) beside b() computes one pass on the frame's world (#4365)

A memo or render effect that reads `latest(a)` beside `b()` computed a
torn pass on a batched write. The `latest()` read of a's staging opened the
flush's transaction for its verdict (verdict.ts `provisionalVerdict`).
The plain `b()` read after it then took that transaction for a hold
(`stagedScreen`): it served b's committed value and made the reader a
stale reader of the transaction. Nothing held the flush, so that torn pass
was one of the flush's frames. Through a memo, a render effect ran `[2,1]`
before `[2,2]`. A throw from the pass stuck, because the landing skips
errored readers.

A pass seated in the verdict lane only by its own `latest()` read
(REACTIVE_PROVISIONAL) now reads a staging as the frame does: it leaves
the ... (continued)

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