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fix(signals): optimistic untracked reads honour #3331 supersession (F3/F5) (#3678)

* fix(signals): optimistic untracked reads honour #3331 supersession (F3/F5)

Two findings from the optimistic list-mutation matrix (#3675), both in
the derived-source shape `createOptimisticStore(() => truth())` that the
router uses.

F5 — key fn called with undefined, REACTIVITY_HALTED. A derived
optimistic store under a keyed `mapArray` (`<For keyed>`) whose truth
lands with a length different from the optimistic frame: the tracked
`length` read (get trap, inline node path -> `serve` -> `supersededRead`)
sees the staged truth because #3331 supersedes the override for a
deriving reader, but the untracked reads mapArray makes inside its
owner (`newItems.slice(0)` under `runWithOwner`) went through the
`serveDataKey` length view, `optimisticView` and the `has` trap's
`visibleOverride`, which return the raw `_overrideValue` and never
consulted supersession. The pass snapshots `_items` one row short and
the next pass hands the key fn `undefined`. Regressed in #3370 (override
supersession, Sep 11), shipped in rc.9. `Object.keys`, `in`, `snapshot()`
and `deep()` lagged by the same row at the landing.

Fix: a `readerOverride(node, committed)` pick — `nodeValue` (the store's
untracked `serve` path, which applies supersession) when the node is
`CONFIG_OVERRIDE_SUPERSEDED`, the raw override otherwise. The `length`
view arm, the `has` trap, `visibleKeys`/`visibleDescriptor` and
`optimisticView`'s reader path (not the write-side `applyTentative` /
`applyAdopt` callers) use it, so every read channel a non-draft reader
has agrees with the tracked one at a landing. Lane/untracked readers
outside the deriving owner keep the override (A17/A18/A29 unchanged).

F3 — stale length override, a hole, under two overlapping actions. The
`length` draft arm fell through to the optimistic view even when the
draft does not see overrides (`draftSeesOverrides(target)` false), so a
second action's ... (continued)

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