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fix(signals): an optimistic guess's replaced row reaches <For> (#4360)

The container's arrangement comparator judged rows by key, keyless rows by
position. A guess that put a different object at an index with the same
key (a fresh `{ id }` row returned from the setter), or any keyless row
(identical rows swapped in the draft, a fresh row with the same contents,
a reorder: #4344), was "the same row": the arrangement guess was dropped
and `<For>` kept the old row's proxy while direct reads served the new
one, so an edit of that row never reached the screen.

A row the store already serves is now its proxy's row: another object at
the index is another row. A landing's object is adopted by the row it
pairs with before the comparator runs, so an echo still confirms (#3898).
Rows no proxy serves yet still pair by key, keyless rows by position.

The comparator also folds its keyless special case into `sameKey`
(`undefined` is the same key), so the store scenario gets smaller.


Claude-Session: https://claude.ai/code/session_015hPxMkC3jg3Uivh65gvaf2

Co-authored-by: Claude <noreply@anthropic.com>

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