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fix(signals): a latest() read of a flight does not hold a Loading on switch (#4661) (#4663)

* fix(signals): a latest() read of a flight does not hold a Loading on switch (#4661)

A memo whose last run read `latest(() => list())` is still the hold's verdict
reader on its next run (D-25). Its read of the switch's own staging (`f`) is
the screen's, and that stale-reader mark (REACTIVE_FRAME_READ) made
`blockedBy` count it as a frame reader of `list`'s new flight too — a flight
it read only inside `latest()`, ahead of it (F-6). The switch was held and
the `on` fallback never showed.

At the seam, a verdict reader that read a flight of the frame only ahead of
it this pass (and no flight plainly: `observeFlight`, `frameRead`) drops the
mark and its rerun on the frame's landing: it holds nothing, keeps its
committed value (maintainer, 2026-11-03: "intentional tearing"), and the
flight's own landing re-derives it. Engine only (lanes.ts, verdict.ts).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0117BoWNSSgtemsSYsaX2hXE

* perf(signals): track #4661's read-ahead readers with one list, no WeakMap

The seam drops the stale-reader mark of a verdict reader whose pass read a
flight inside latest() before any plain flight read routed it (F-6). A plain
read of a flight first routes the pass (REACTIVE_LANE_READ), so the read-ahead
push is skipped and the reader still holds (A10, A15); later plain reads in a
routed pass are the lane's and mark nothing. That replaces the pass-generation
tuples, the plain-read WeakMap and the laneServed hook. A new variant pins
the plain-read-first hold.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0117BoWNSSgtemsSYsaX2hXE

* fix(signals): a memo's plain read of a flight after its latest() window still holds (#4661)

A window read routes the pass into the verdict lane, and `read` then served a
later plain read of a flight... (continued)

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