Tidy up sumcheck RoundCoeffs/RoundProof and add tests

[tcoratger]

Cleans up the sumcheck round-polynomial helpers and gives the module its first tests.
Pure refactor — no protocol change, no behavior change at any call site.

The problem

RoundCoeffs / RoundProof are the round-polynomial types the sumcheck and MLE-check verifiers use.
Their methods were split across three different trait bounds with no stated reason:

• evaluate and recover required FieldOps.
• The endpoint helpers, the Add/Mul/Sum operators required the narrower Field.

The only thing the narrower bound bought was Copy.
So a round polynomial over a packed field could be evaluated but not added or scaled — an arbitrary, undocumented restriction, and a seam inconsistent with the mlecheck twin which already commits to FieldOps.

Two smaller issues rode along:

• The derived Default demanded $F$: Default even though the type just wraps a Vec, which is always Default.
• The module had zero tests, while its mlecheck sibling tests the same round-trip.

The idea

Unify every method on FieldOps — the more general bound the abstraction was built for (packed and symbolic fields), and the one evaluate/recover already require.
$F$: Field still satisfies $F$: FieldOps via the blanket impl, so every existing scalar call site is unaffected.

The change

- use binius_field::{Field, field::FieldOps};
+ use binius_field::field::FieldOps;

- impl<F: Field> Add<&Self> for RoundCoeffs<F>      // and Mul, Sum, endpoint helpers
+ impl<F: FieldOps> Add<&Self> for RoundCoeffs<F>

• Default: replaced the over-constrained derive with a manual impl<F> Default on both types, so they are Default for every $F$ (also required once the bound moved to FieldOps, which does not imply Default).
• DRY: extracted a private endpoints() -> (F, F) to remove the duplicated (R(0), R(1)) prologue shared by the two endpoint helpers.
• Cleanup: dropped a redundant where F: FieldOps on recover; added an as_slice accessor to retire the self.0.0 double-tuple in coeffs.
• Docs: documented the two silent contracts — truncate's non-empty precondition, and why recover subtracts the constant term twice (the two copies cancel in characteristic 2 but keep the identity correct over any field).

Soundness

Nothing changes on the wire.
The bound widening is a pure generalization — every scalar caller resolves to the identical code.
The recover arithmetic is byte-for-byte the same; only its doc and a redundant bound changed.

Scope

• changed: crates/ip/src/sumcheck/common.rs only.
• added: a tests module (8 tests) — recover round-trip, the s = R(0) + R(1) identity, both endpoint helpers, ragged-length addition, scaling, iterator Sum, and the empty/zero-polynomial edge case.
• untouched: every call site; the mlecheck twin (its truncate API-shape divergence is left for a separate change).

Verification

• cargo check -p binius-ip --all-targets — clean.
• cargo clippy -p binius-ip — clean.
• cargo +nightly fmt --all — clean.
• cargo test -p binius-ip -p binius-ip-prover — 70 passed, 0 failed (8 new).

Note: cargo check --workspace --all-targets currently fails on an unrelated pre-existing error in crates/ip/src/channel.rs (call_native on FieldFn) that reproduces on a clean main under full-workspace feature unification — it is not introduced by this PR.

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