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fix(signals): a fold repeats no unchanged presence to a held node (#3743, supersedes #3744) (#3791)

* fix(signals): a fold repeats no unchanged presence to a held node (#3743)

Port of #3744 (brenelz) onto L2 (#3774). The fold's presence loop
(`notifyFoldTail`) wrote every observed `in` node with `key in neu`;
`setSignal` joins a held node's transaction before its equality gate
(A34 (1): a write to a held node is a proposal, the same value or
another), so a `reconcile()` outside an action that had deleted an
observed key — the snapshot leaving it absent too — made the whole
mainline tick the action's: an unrelated `a.value` stayed stale until
the action settled. Presence is now diffed `old` -> `neu` like the
leaves (the fold's contract, #3296: the view the nodes were last told);
a live chained `old` (a store proxy) is written unconditionally as
before. `notifyFold` shares the tail.

`applyAdopt` materializes a nested overlay draft before taking its diff
base (`adoptPB` did so after it was taken): through the overlay's
prototype a deleted key still read as present, so the presence skip
fired on a real deletion, and a reconcile restoring the key never
re-proposed on the held leaf, which committed the draft's `undefined`
beside a backing that had the key.

Two L2 fixes the diff uncovered, pinned by the existing #2719/#3164 and
Q-D twin tests and a new leaf-only reader case: a container carrying an
arrangement guess is told of an arrangement change whether or not
anything subscribes to it (`notifyFoldTail`/`notifyWrites`) — for a
guessed container the write is the landing that judges the guess
(Q-D, plan sec. 39), and before only the unconditional presence write
reached the lane, so a newer question's rows landing beneath an
optimistic push published beside it for a reader of leaves alone; and
`_laneRebase` no longer re-stages a slot whose row the committed
backing already shows by key, nor an unchanged `length` (a spurious
identical frame).

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