Draft PR. Please ignore until reviewed by @ChrisRackauckas.

Summary

• Add reshape(::DiffCache, dims...) so vector-backed DiffCache storage can be presented as a multidimensional cache without callers reaching into .du/.dual_du.
• Keep the reshaped normal cache view-backed so the original vector cache can still be resize!d and reshaped again.
• Preserve the existing raw dual-storage ratio when resizing vector-backed DiffCaches.
• Keep vector-backed reshaped dual temporaries zero-copy by detecting vector/subvector-backed reshapes through public array APIs before falling back to ArrayInterface.restructure.
• Document the resize/reshape workflow and bump the version to 1.3.0 for the new public API.

Root Cause

The issue workflow needed multidimensional DiffCache access while keeping resizeable vector storage. A plain reshape(vector, dims...) marks the vector as shared and prevents later resize!, while the existing public API did not expose the field/unsafe_wrap pattern safely.

The initial implementation used a view-backed reshape for safety, but the ForwardDiff path for that wrapper fell back to ArrayInterface.restructure, which copied into a fresh matrix. The current implementation adds a vector-backed reshaped-cache fast path so get_tmp remains zero-copy and writes alias the raw dual storage, without relying on non-public Base.ReshapedArray in source.

Fixes #140.

Performance Notes

Benchmarked locally with BenchmarkTools after rebasing onto current master, on a 1000-element vector-backed cache reshaped to (20, 50):

• get_tmp for reshaped dual cache: 23.63 ns, 0 bytes
• ordinary matrix dual cache get_tmp: 23.63 ns, 0 bytes
• reshape(DiffCache vector): 5.63 ns, 0 bytes
• unsafe_wrap(Array, pointer, dims): 45.35 ns, 48 bytes
• elementwise fill/sum on the reshaped cache matched unsafe_wrap and ordinary matrix timings and allocated 0 bytes
• reshaped normal and dual temporaries are StridedArrays and the dual temporary aliases dual_du

Non-standard probes:

• SubArray and LVector vector-backed reshapes return strided, zero-copy reshaped dual temporaries.
• LArray matrix reshapes stay on the existing ArrayInterface.restructure fallback, preserving wrapper behavior instead of applying the vector-backed fast path.
• VectorOfArray reshaped normal access remains zero-allocation.

Validation

• timeout 3600 /home/crackauc/.juliaup/bin/julia +1.10 --project=. test/core_resizing.jl
• timeout 3600 /home/crackauc/.juliaup/bin/julia +1.10 --project=. -e 'using Runic; Runic.main(ARGS)' -- --check --diff .
• timeout 3600 /home/crackauc/.juliaup/bin/julia +1.10 --project=. -e 'using Pkg; Pkg.test()'
• GROUP=QA timeout 3600 /home/crackauc/.juliaup/bin/julia +1 --project=. -e 'using Pkg; Pkg.test()'
• Docs build initially hit a transient GitHub 429 during Documenter linkcheck; retrying the same command unchanged passed: timeout 3600 /home/crackauc/.juliaup/bin/julia +1.10 --project=docs -e 'using Pkg; Pkg.develop(PackageSpec(path=pwd())); Pkg.instantiate(); include("docs/make.jl")'
• BenchmarkTools probes in a temporary local perf_env, removed after use.