Support DiffCache for nonzeroable element types

[ChrisRackauckas-Claude]

Summary

• allow DiffCache backing storage to fall back to uninitialized storage when zero(T) is unavailable
• preserve zero-filled backing storage for element types that still support zero(T)
• add a regression test for an isbits named-tuple element with nested tuple fields under ForwardDiff dual replacement

Motivation

DiffCache(ar_alloc) in #71 failed in the constructor because zeros(T, ...) requires zero(::Type{T}). Named tuples do not provide that method even when they are isbits and usable as cache elements.

Validation

• Reproducer for a Vector of named tuples now constructs DiffCache and prints the expected cache length and element type.
• timeout 3600 /home/crackauc/.juliaup/bin/julia +1.10 --project=. -e ... Pkg.test() passed.
• Runic.main(["--check", "."]) on Julia 1.10 passed.

This PR should be ignored until reviewed by @ChrisRackauckas.

Fixes #71.