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feat(signals): A28 — a write becomes visible at flush, to every channel (read-side) (#3473)

Between set(x) and the flush that carries it the write is unflushed: not the
committed value, not the staged value latest()/isPending() serve, and not an
input to a derivation created meanwhile. Optimistic writes match (A28 (5)):
setOptimistic(v) becomes the active override at the carrying flush; the
writer's own channels compose on it. A rewrite of a held node keeps the
staged value the last flush left for the verdict channels until the next
flush. Companions and store keys first materialized under a hold are born
as the holding transaction's (#3336).

Landed as a read-side rule rather than #3337's deferred subscriber walk:
"unflushed" is structural (an ambient staged value outside a flush), read
sites test one module flag (`unflushedStaged`, set when a node is staged or
a held node rewritten outside a flush, cleared at flush start), and the
write-path arms are cold helpers gated on loads the write already pays
(`_transition`, `context`) — setSignal stays within every setter's inlining
budget (~360 B bytecode; the arms inline cost 140 B and 10–20% on the
write-loop benches). Readers served the flushed value are latched for the
carrying flush (REACTIVE_MISSED_WAKE).

A companion created lazily while its source carries an unflushed write joins
the flush-start re-sync like one that existed at the write, so a derivation
over latest() direct-commits as the optimistic view it is rather than being
staged under whatever hold the round entered (a memo — and an effect — over
latest() of a held source created mid-tick answered the previous value, or
never ran, until the fetch settled).

Supersedes the #2922 mid-tick latest() pull (flush() first to read your own
write); OL-R2 / OS-R1 / CS-R34 superseded. Spec: A28 ruled + mechanism,
A29 creation-time form, A7 amendment (latest() throws in every uninitialized
scope). Oracles: every pre-flush cell reads the rule; no violat... (continued)

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