feat(TextureIGListKitExtensions): add performUpdatesWithFallback with pre-flight section-count guard

[3a4oT]

Summary

Adds ListAdapter.performUpdatesWithFallback(animated:completion:) (plus an async overload) that performs a pre-flight check UICollectionView.numberOfSections == adapter.objects().count and falls back to reloadData(completion:) on divergence, instead of letting IGListKit raise NSInternalInconsistencyException from IGListBatchUpdateTransaction._didDiff:onBackground: with

The UICollectionView's section count (N) didn't match the IGListAdapter's count (M), so we can't performBatchUpdates. Falling back to reloadData.

(IGListKit's message text says "falling back to reloadData", but IGAssert raises an NSException in release builds — the documented fallback never executes.)

Scope

Catches already-divergent states, where the collection view was reset externally (reloadData from another path, data source reassignment, view re-layout that lost its section count) while the adapter still holds the previous applied snapshot.

Out of scope

Does not prevent the same exception when triggered by a concurrent in-flight diff: IGListKit captures the sectionMap inside its background diff queue, and a second performUpdates that mutates the data source between snapshot and apply still raises from _didDiff:onBackground:. Callers must serialize concurrent loads on the same adapter — this guard is a defence-in-depth layer, not a replacement for caller-side serialization.

Also out of scope: ASCollectionInvalidUpdateException raised from ASDataController.mm:598 when a changeSet validates against a data source already mutated by a subsequent load. That race requires caller-side serialization of the load → performUpdates cycle.

Tests

IGListAdapterBridgeTests adds:

• performUpdatesWithFallback_appliesDiffs_whenSectionCountsMatch — happy path, ensures the guard does not regress normal consecutive diffs.
• performUpdatesWithFallback_isNoOp_whenNoCollectionViewAttached — verifies the nil-collectionView fall-through to performUpdates.

The fallback branch is not exercised by a dedicated test because the IGListKit + AsyncDisplayKit API contract intentionally prevents callers from constructing the divergent precondition (dataSource swaps crash on interop selectors; IGListAdapter is not open for subclassing outside its defining module; method-swizzling UICollectionView.numberOfSections would leak across the surrounding regression tests). The branch is one if statement and is verified by inspection; an inline comment in the test file documents this rationale.

Local run on iPhone 17 / iOS 26.4.1 simulator: 9/9 tests passed (7 existing regression + 2 new).

Observability

Divergence events are logged via os_log at .error level under subsystem: TextureIGListKitExtensions, category: iglistkit-guard. Consumers can filter for these in Console.app or pipe them into their structured-logging stack to measure how often the fallback fires in production.

Test plan

• Verify CI passes on the SPMWithIGListKit test scheme (iOS Simulator).
• Confirm public API addition is acceptable (new method on ListAdapter) — no breaking changes to existing API.
• Tag a patch release (e.g. 3.3.5) once merged so SPM consumers can pin the new revision.