Don't run a memoize's producer on a model torn down mid re-evaluation

[mansbernhardt]

Fixes the unattributed memoize(...) on an unanchored model node is not allowed issues reported from parallel-apple's ParallelEditorTests. That suite records 57 of them per run, so xcodebuild exits nonzero even though the log says TEST SUCCEEDED, which blocks their PR #2407.

Mechanism

• A teardown cuts a model's parent links before it destructs the model and cancels its memoizes, and that parent-link change wakes memoizes that read ancestors.
• On the withObservationTracking path the re-evaluation runs on the background queue. One already past its hasBeenCancelled check blocks on the context lock until the cascade finishes, then runs produce() on a model with no ancestors.
• The downstream accessor node.memoize { node.member(in: .ancestors) ?? TimelineModel() } then falls back to a fresh, never-anchored TimelineModel. A memoize on that model (mediaModel(for:)) reports from the background queue after the test has finished, so the issue is attributed to no test.

Fix

Narrow and local. Once inside the context lock, the memoize update closure checks whether the model has been destructed. If it has, it serves the last produced value instead of running the producer. The result would have been discarded anyway, because teardown clears the cache.

Evidence

MemoizeDuringTeardownTests reproduces the downstream shape: a child memoize resolves an ancestor, falls back to a fresh model, and calls a memoize on the result.

• Without the fix: over 200 iterations the producer ran on a torn-down model 293 times and recorded 186 unattributed issues (ModelNode.swift:551, the downstream signature).
• With the fix: 0 and 0. The test now runs 60 iterations, where the failure without the fix would still show roughly 90 fallbacks.
• Full suite: 3 parallel runs and 1 serial, all green, with no compiler warnings. Its only remaining ModelNode.swift:551 hits are the deliberate withKnownIssue tests of memoize on a never-anchored model.

CHANGELOG entry added under [Unreleased].

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