Fix CollectionViewLayout delegate lifetime

[RoyalPineapple]

Problem

CollectionViewLayout held its delegate as unowned let. In normal use the ListView ownership graph keeps the delegate alive for the layout's whole lifetime, but a layout swap (LayoutManager.set(layout:)) or test setup that constructs a layout in isolation can deallocate the delegate first, leaving a dangling unowned reference and a hard crash on next access.

Fix

Change delegate from unowned let to private(set) weak var, and tighten the surrounding call sites:

• prepare() reads the delegate once at the top, returns early if it's gone, and threads the unwrapped reference through performLayout, performRebuild, and performLayoutUpdate to avoid per-call weak loads.
• init keeps a non-optional delegate parameter; the weak storage already conveys "may vanish later," and the parameter keeps construction honest.
• LayoutManager.set(layout:) uses listableInternalFatal if the outgoing layout's delegate has been deallocated, since silently dropping a layout swap is worse than crashing on a "should never happen" state.
• sendEndQueuingEditsAfterDelay() snapshots the delegate locally and weak-captures it, so the closure neither pins self nor extends the delegate's lifetime past its natural end.

Test

Adds CollectionViewLayoutTests.test_prepare_whenDelegateHasBeenDeallocated, which constructs a layout with a mock delegate, drops the strong reference, asserts the weak references are nil, and calls prepare() to verify it doesn't crash.

Verified locally that the test fails against origin/main's source and passes with this PR's fix.

Before (against origin/main's unowned let delegate):

Test Case '-[ListableTests.CollectionViewLayoutTests test_prepare_whenDelegateHasBeenDeallocated]' started.
Fatal error: Attempted to read an unowned reference but object 0x1027ed7a0 was already destroyed
** TEST FAILED **

After (this PR):

Test Case '-[ListableTests.CollectionViewLayoutTests test_prepare_whenDelegateHasBeenDeallocated]' started.
Test Case '-[ListableTests.CollectionViewLayoutTests test_prepare_whenDelegateHasBeenDeallocated]' passed (0.001 seconds).
** TEST SUCCEEDED **

Risk

Low. ListView retains its Delegate for its entire lifetime, so in normal flows the delegate is alive every time the layout uses it. The fix is defense-in-depth for layout swaps and isolated-construction paths.

Checklist

• Ensure any public-facing changes are reflected in the changelog. Include them in the Main section.