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fix(signals): lanes hold on observed async only, like transactions (#3289)

A lane (optimistic write or latest() read) revealed ahead of its
transaction but held that reveal on ANY async derived under it — a memo
nobody rendered, or one a fallback-showing Loading boundary had caught,
blocked the lane exactly like a torn frame would. Transactions never
worked that way: an async blocker enters a transaction only from queue
notification, when a render effect observes it pending and no boundary
consumed the status (INV-3).

Lanes now use the same rule. `laneHeld` holds a lane iff some node in its
`_pendingAsync` (the async the lane owns, registered at recompute as
before) is also in its transaction's `_asyncReporters` (the async a
render effect observed). The two facts can arrive in either order — a
memo created by the lane's own reveal is observed first and lane-stamped
on a later re-ask, which sends no fresh notification — so the hold is a
predicate over both records rather than a registration at either site.

Spec: lane-hold-on-observation.test.ts, both lane kinds — observed
derived async holds; unobserved does not; an on:-reset boundary does
not; a created-under-lane re-ask holds. No existing test changes.

Size: isPending/latest 10.08 -> 10.13 KB (measured 10.080; ~40 B in the
lanes module this scenario retains). Core floor and createStore
unchanged.

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

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