`ResizableArray` iteration throws `BoundsError` on ragged/sparse tensors

[docxology]

Summary

Base.iterate for ResizableArray (src/resizable_array.jl:175-194) indexes by the
reported (= max-extent) size without an isassigned guard. Iterating a ragged or
sparsely-filled tensor (collect, for … in arr, map, broadcast) raises BoundsError
for unassigned slots. Base.vec/first/firstwithindex/lastwithindex do guard with
isassigned, so iteration is the odd one out.

Repro

using GraphPPL
a = GraphPPL.ResizableArray(Float64, Val(2))
a[1, 1] = 1.0; a[1, 2] = 2.0; a[2, 1] = 3.0   # row 2 has no [2,2]
size(a)                    # (2, 2)  — max extent
isassigned(a, 2, 2)        # false
collect(a)                 # ERROR: BoundsError
GraphPPL.vec(a)            # [1.0, 3.0, 2.0]  (guarded — works)

Suggested fix

Guard iterate (both the 0-arg and state-carrying methods) with isassigned, skipping
unassigned slots — consistent with vec/first. Alternatively document that full iteration
requires a dense array and steer users to vec.