fix: guard ResizableArray.iterate — skip unassigned slots, don't crash on empty/sparse tensors

[docxology]

Description

Fixes the sparse/ragged-tensor and empty-tensor iteration crashes in
src/resizable_array.jl:

• sparse/ragged tensors (unassigned slots): for e in a threw BoundsError because
  iterate indexed by the reported max-extent size without an isassigned guard;
• empty tensors: iterate(array) destructured iterate(CartesianIndices(...))
  unguarded, crashing on nothing.

Change

Rewrote the two Base.iterate methods to skip unassigned slots and to guard the
exhausted/empty cases, delegating to a small helper _resizable_iterate_step
(see findings/finding-05-…md for the exact code). Dense iteration order and the standard
zip/enumerate iteration protocol are unchanged.

Tests

Regression test in test/resizable_array_tests.jl: sparse ResizableArray (unassigned
slot) iterates via for/zip/vec without throwing and yields only the assigned values;
an empty ResizableArray iterates to zero elements.

Verification (Julia 1.12, direct)

check	main	this branch
dense for e in s order == vec(Array)	pass	pass
sparse 2-D hole: for e in a	BoundsError	pass ([1,2,3])
empty 2-D: for e in empty	threw	pass (0 elems)
zip(Array′, s) order	pass	pass

Note: the full test/resizable_array_tests.jl run via ReTestItems is blocked on this box by
the pre-existing GraphViz weak-dep precompile failure on Julia 1.12 (minor-06), unrelated to
this change; the iterate item itself runs and the direct checks above mirror its assertions.