Thread ForwardDiff chunk size through FunctionWrapper compilation

[ChrisRackauckas-Claude]

Summary

• Add _forwarddiff_chunksize helper to read the new SciMLBase.forwarddiff_chunksize trait
• Thread Val(CS) through get_concrete_problem → promote_f → wrapfun_iip for both ODE and DAE problems
• Add parameterized dualgen(T, Val(CS)) in the ForwardDiff extension
• Add 3-arg wrapfun_iip(ff, inputs, Val(CS)) that compiles FunctionWrapper variants with chunk=CS for u-related duals (Jacobian) and chunk=1 for t-related duals (time derivative)
• Add 3-arg fallback wrapfun_iip(ff, inputs, ::Val) in norecompile.jl for when ForwardDiff extension isn't loaded

Context

DiffEqBase PR #1284 enabled FunctionWrapper wrapping for mass matrix ODEProblems. The FunctionWrapper variants were compiled with Dual{..., 1} (chunk=1 hardcoded in dualgen). When algorithms use a different chunk size (e.g. AutoForwardDiff(chunksize=3)), the solver creates Dual{..., 3} values but the wrapper only has Dual{..., 1} variants, causing NoFunctionWrapperFoundError.

The key insight is that u-related duals (Jacobian computation) use the algorithm's chunk size, while t-related duals (time derivative) always use chunk=1 since it's scalar differentiation. The 3-arg wrapfun_iip handles this by creating variants with different chunk sizes for the u-derivative vs t-derivative paths.

Depends on: SciML/SciMLBase.jl#1244

Test plan

• Precompilation succeeds with the changes
• Val(1) default preserves existing behavior when trait returns 0
• Tested end-to-end with OrdinaryDiffEqRosenbrock DAE AD tests (8/8 pass)
• Rosenbrock convergence tests still pass (853/853)
• CI

🤖 Generated with Claude Code

[ChrisRackauckas-Claude]

Updated to dispatch on Val from the trait directly instead of runtime branching.

Now that SciMLBase.forwarddiff_chunksize returns Val{N}(), the helper becomes:

_forwarddiff_chunksize(::Nothing) = Val(1)
_forwarddiff_chunksize(alg) = _resolve_chunksize(SciMLBase.forwarddiff_chunksize(alg))
_resolve_chunksize(::Val{0}) = Val(1)
_resolve_chunksize(v::Val) = v

No runtime Int comparison or Val() wrapping needed. Call sites simplified from Val(_forwarddiff_chunksize(alg)) to _forwarddiff_chunksize(alg).

[ChrisRackauckas]

locally handled.