refactor(fs): return named structs from fold APIs

[junbyjun1238]

This PR replaces tuple-returning fold outputs with named structs and updates the relevant CycleFold call sites to use the new named results.

This is a breaking API cleanup that makes fold artifacts explicit by name across the folding-scheme interfaces.

Note to reviewers: Because this replaces the signatures of the canonical folding traits, the migration necessarily updates the current fs implementations (Nova, HyperNova, Mova, Ova, and ProtoGalaxy) and the downstream CycleFold consumers. Most of the resulting churn is mechanical.

Why:

• The CycleFold path still passes large positional tuples like (RW, RU, Proof, Challenge) and (RU, Challenge) through some of its most complex code paths.
• Naming those artifacts makes these call sites easier to read and review.

What changes:

• change FoldingSchemeProver::prove(...) to return FoldStep
• change FoldingSchemePartialVerifierGadget::verify_hinted(...) to return PartialVerifierStep
• migrate the existing Nova, HyperNova, Mova, Ova, and ProtoGalaxy implementations to the new return types
• update the relevant CycleFold call sites to consume the new named structs

Testing:

• cargo test -p sonobe-fs
• cargo test -p sonobe-ivc

References:

• prove returns the challenge explicitly for CycleFold:
  crates/fs/src/definitions/algorithms.rs#L33-L59
• verify_hinted is intentionally a partial verifier:
  crates/fs/src/definitions/circuits.rs#L9-L35
• Current tuple plumbing in the CycleFold prover path:
  crates/ivc/src/compilers/cyclefold/mod.rs#L248-L312
• Current tuple plumbing in the augmented-circuit path:
  crates/ivc/src/compilers/cyclefold/circuits.rs#L118-L145