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fix(signals): an optimistic draft sees the row a previous setter added in the same action (#3665) (#3670)

Inside an action, a second optimistic-store setter in the same synchronous
stretch saw the first setter's structural add on `length` and `in` but read
`undefined` for its value — `d.push(row)` then `d[d.findIndex(...)]` missed.
The draft is the writer's channel and composes on the tick's own unflushed
overrides (`hasActiveOverride`'s rule — the has trap's draft arm,
visibleKeys, optimisticView and ensurePB's seeding all apply it); rc.9's A28
sweep (8ff4803) reader-gated the one remaining draft arm, the get trap's
absent-own-key arm, on `visibleOverride`. Restored to the writer rule.

Next to it, `visibleDescriptor` had no draft arm at all (since ae0ec3f5,
rc.8): `ownKeys` listed the added key while its descriptor reported it
absent, so every enumerator — Object.keys, spread, Object.entries,
JSON.stringify — dropped the row inside a later setter, and on arrays rc.9
serialized a literal `null` hole. It gains the twin of visibleKeys' split
(`draft ? hasActiveOverride : visibleOverride`, gated on
draftSeesOverrides), reading the synthesized descriptor's value from the
override itself rather than through nodeValue (the reader rule).
snapshotWalk — the value deep()/snapshot() return — composes the same way
inside a draft, and skips composition over a draft's own view-seeded
backing (composing again would clobber the draft's later writes with the
overrides they superseded).

Reader-side visibility (A28 (5)) does not move: an effect and an untracked
read see the add only at the flush that carries it, and the revert follows.
#3668's enumerator skip is untouched — inside a draft there is no observer
and no presence read; the new arm births no nodes.

Tests: tests/store/optimistic-draft-visibility-3665.test.ts — every channel
in the reports' tables (get / has / length / ownKeys / descriptor /
Object.keys / spread / Object.entries / JSON.stringify / deep()) ... (continued)

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