[2.0 / next] Loading on={[latest(id), filter()]} re-arms ahead when filter is held #4698

Ruled-not-built row S-49 from #4695. The ruling is in packages/web/test/async-gen/rulings/S-49.ts. Rule: D-17, as restated by Decision 1 (posture is per read) in documentation/plans/optimism-seam-spike-brief.md §8.

Shape: <Loading on={[latest(id), filter()]}>, where one action writes both id and filter.

Ruled: filter() is a plain read of a held staging, so the on pass is the hold's work. The re-arm waits for the frame, and the swap lands when the hold commits. A re-arm goes ahead only if every held value the on expression read, it read ahead.

Today: the fallback shows right away (vm0:2 [L]), because one probe makes the whole on pass a verdict reader.

This changes behavior against today on purpose, as flagged in the brief.