[huashengshao]

Add fresh native preparation and evaluation of coherent bare one-loop amplitudes for color-singlet scalar theories with momentum-independent local numerators. FeynKit owns diagram generation, model and symmetry factors, momentum routing and quadratic-pole extraction; exact coefficient instructions feed the existing NinjaRS/OneLOop reduction path. All complex diagram amplitudes are retained and summed before any Born contraction.

ScalarLoopProgram accepts typed models/processes and exact parameter specialization. Internal poles must be monic simple zero-width vacuum poles with real bound masses. Repeated poles, derivative/tensor/dimension-dependent numerators and nontrivial projectors fail explicitly. Tadpoles and zero-flow bridges remain present unless the caller explicitly filters them; singular evaluations never expose partial diagram sums. There are no process-specific branches, Python execution or process-specific compilation.

Outputs are complex Laurent coefficients in the existing MadLoop/BLHA C-epsilon-stripped bare convention, including i/(16*pi²). They do not include UV counterterms or infrared remainder conversion: the finite entry is not an NSC R1, and its square is not a finite one-loop norm. Flux, external-state averages and final-state phase-space factorials belong to consumers. Callers own signed momentum conservation and any scattering on-shell checks; off-shell Green functions are supported.

The explicit-bit evaluator constructor passes the same working precision through coefficients, normalization, sampling and reduction, using the existing native carriers. It extends PolynomialPlan with the same bit-precision contract as the coefficient evaluator. Cancellation may reduce tracked reliability; no decimal round-trip or hidden guard bits are introduced.

Validation: ten native scalar-loop tests pass, covering analytic complex tadpole/bubble Laurent coefficients, distinct masses and coherent sums, retained bridges, unsupported poles/numerators, ordinary nonzero masses named ZERO, exact 2^-80 momentum sensitivity at 173 working bits, cross-lane precision isolation, and malformed/empty input recovery. The prebuilt binary passed the same ten tests with PATH=/nonexistent, preparing fresh models/processes without Python or external compiler lookup. Source review independently checked routing, normalization, factors, precision and failure handling.

cargo test --offline --locked -p rusticol-loop-reduction scalar_loop:: --lib -j2 -- --test-threads=1

The tested dependency pair is FeynKit 4b8569c0, NinjaRS de858c88, OneLOop 8ec256fc and Symbolica/Numerica/Graphica 9cdc2949. This PR stacks on the separately validated native-provider integration branch; broader QCD-loop preparation and renormalized finite remainders remain separate capabilities.