test(wide_math): property-based algebraic coverage for U256/I256 and spec 4.6/4.8 helpers

[gusha625]

Why

src/wide_math.rs hand-rolls 256/512-bit arithmetic twice (Kani [u128;2] mode, BPF [u64;4] mode) with no external big-int crate to cross-check. Every haircut ratio, funding accrual, and ADL settlement flows through it. The in-module tests pin known values; nothing pinned the algebra under randomized inputs on the host build.

What

tests/wide_math_props.rs (proptest, ~13k random cases per run, 17 test functions):

1. Ring identities -- add/sub roundtrips, commutativity, saturating boundaries, Ord vs limbwise comparison.
2. Differential multiplication -- checked_mul and the private U512 path behind the mul_div_* family compared limb-by-limb against an independent 4x4 u64-limb schoolbook reference (deliberately a different decomposition than the u128-half implementation under test), including the exact overflow boundary.
3. Euclidean division -- div_rem_u256 satisfies q*d + r == n, r < d, verified with the reference multiplier rather than the engine's own checked_mul.
4. Spec 4.6 floor/ceil duality -- ceil == floor + remainder indicator across the U256 and u128 variants; signed floors agree with native i128::div_euclid on the shared domain.
5. Spec 4.8 K-pair settlement -- wide_signed_mul_div_floor_from_k_pair and wide_signed_mul_div_floor match a native i128 model on ranges where the product provably fits.
6. Deterministic edge pins -- i128::MIN fee debt, I256::MIN negation, division at ONE/self/MAX, 2^255 shift boundary, checked quotient overflow.

One boundary finding, documented in the test rather than changed: floor_div_signed_conservative_i128(i128::MIN, 1) hits its out-of-range guard even though the final result is representable. Unreachable in production (engine callers divide by FUNDING_DEN / POS_SCALE >= 10^6); the U256 variant handles the point exactly and is tested on the full domain.

Gated on fork-facade (wide_math is only pub when the wrapper opts in); registered as a [[test]] target next to v16_fuzzing.

Run

cargo test --features fork-facade --test wide_math_props

Full suite with fork-facade: 151 passed, 0 failed. Default-feature suite unchanged.

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Reviewing files that changed from the base of the PR and between d1345d7 and bd431e2.

📒 Files selected for processing (2)

• Cargo.toml
• tests/wide_math_props.rs

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