Perf #8: Verify precomputed NamedTuple is already type-stable

[hmzh-khn]

Summary

Investigation for Perf #8 (concrete type for NonlinearSolver.precomputed NamedTuple)
found that no refactor is needed — the NamedTuple approach is already fully type-stable.

Investigation Findings

The task hypothesized that the TC<:NamedTuple type parameter in NonlinearSolver{TP, TC}
could cause inference issues when the solver is passed through generic functions. Empirical
testing with @code_warntype and @inferred proved this is NOT the case:

1. isconcretetype(typeof(solver.precomputed)) returns true — the NamedTuple type
   is fully concrete with all field types known at compile time
2. All 11 fields in the precomputed NamedTuple have concrete types
3. @inferred passes for field access patterns used in solve_raw and run_nonlinear_solver
4. Julia specializes on concrete NamedTuple types regardless of ::NamedTuple annotations
   in function signatures (as in run_nonlinear_solver)

Why no refactor is needed

NamedTuple field access is resolved at compile time when the concrete NamedTuple type is
known. Since NonlinearSolver{TP, TC} carries the full TC type parameter, any function
taking a NonlinearSolver will be specialized on the concrete NamedTuple type. The
::NamedTuple annotation in run_nonlinear_solver is just a type constraint — Julia still
dispatches and specializes on the concrete type of the argument.

Replacing the NamedTuple with a concrete struct would add code complexity without any
performance benefit.

Changes

• test/test_type_stability.jl: Added 15 new tests in "NonlinearSolver.precomputed NamedTuple is type-stable" testset:
  • Verifies solver and precomputed types are fully concrete (isconcretetype)
  • Verifies every precomputed field has a concrete type
  • Verifies @inferred passes for field access from NonlinearSolver
  • Verifies @inferred passes through ::NamedTuple annotation dispatch

Testing

• All 1356 tests pass (julia --project=. -e 'using Pkg; Pkg.test()')
• New tests specifically validate type stability with @inferred
• No source code changes — tests only

Changelog

• 2026-02-16: Initial PR — investigation proves NamedTuple is type-stable, adds guard tests