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fix(signals): a projection draft is valid until superseded or disposed, not until no longer in flight (#3585) (#3595)

* fix(signals): a projection draft is valid until superseded or disposed, not until no longer in flight (#3585)

The draft handed to a createProjection / createStore(fn) / createOptimisticStore(fn)
run now stays writable after the derive returns until the next run of the derive
starts or the owner is disposed (proj R37). A subscription set up inside a sync
derive can push into the draft from its callback without the never-resolving-
Promise workaround.

The old gate (`!settled || owner._x?._inFlight === result`) was timing-dependent:
a sync derive's `undefined` result equalled a never-installed `_x`'s `_inFlight`,
so the "dead" draft was live exactly while nothing had read the projection, and a
superseded run's leaked callback kept writing into the next run's state. The gate
is now a per-run token on the family plus `isDisposed(owner)`.

A late write with no flight up arms the flush itself: `schedule()` withholds its
microtask under projectionWriteActive (a landing normally drains those writes and
a drain before the landing tears the run), and the draft re-arms it through
`scheduleWithheld` — before, such a write left `scheduled` set with no microtask
and stranded the whole scheduler until an explicit flush().

Writes to a superseded or disposed draft are dropped silently (same as R26).
seedLoadingValue's detached shadow is a documented carve-out.

Co-authored-by: Claude via Cursor <noreply@cursor.com>

* fix(signals): a projection run's own draft writes arm the flush they withheld (#3585 follow-on)

A derive body only runs outside a flush at creation. Its own draft writes
made schedule() withhold the microtask (projectionWriteActive) and, for a
sync derive, nothing ever re-armed it: a projection created in a top-level
createRoot left `scheduled` set with no microtask, so its root's effects and
every unrelated write anywhere waited fo... (continued)

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