Handle isbits wrapper eltypes in DiffCache

[ChrisRackauckas-Claude]

This PR should be ignored until reviewed by @ChrisRackauckas.

Summary

• Preserve isbits wrapper eltypes for DiffCache when the active ForwardDiff input is a dual type, e.g. Complex{Float64} now maps to Complex{Dual} instead of a plain Dual array.
• Generalize the behavior through recursive type-parameter replacement, including promoted numeric parameters like ComplexF32 with a Float64 dual.
• Add regression coverage for the issue MWE, a custom isbits wrapper, and ComplexF32.

Closes #47.

Local verification

• timeout 3600 /home/crackauc/.juliaup/bin/julia +1.10 --project=. -e 'using Runic; exit(Runic.main(ARGS))' -- --check .
• timeout 3600 /home/crackauc/.juliaup/bin/julia +1.10 --project=. -e 'using Pkg; Pkg.test()'
• timeout 3600 env GROUP=QA /home/crackauc/.juliaup/bin/julia +1.10 --project=. -e 'using Pkg; Pkg.test()'