Refactor/codec back to spongefish

[ricardo-perello]

Summary

Reverses the codec inversion that shipped in PR #1 of this fork. After this change, codec traits live in spongefish (where they belong), and the DSFS compiler bridging spongefish transcripts with the Argus ia-core IA/NIA traits lives in a new sibling crate spongefish-dsfs.

Why

PR #1 moved DSFS into core spongefish and, to break the resulting dependency cycle, moved codec traits (Encoding, Decoding, NargSerialize, NargDeserialize) into Argus ia-core. This left spongefish with a backwards dep on ia-core purely to import codec types it should own.

The fix is the dependency direction the project actually wants:

spongefish (core)         ── no Argus dep
ia-core (Argus)           ──▶ spongefish core   (re-exports codec traits)
spongefish-dsfs           ──▶ spongefish core
                          ──▶ ia-core           (for IA/IR/NIA/NIR traits)

What changes

New crate: spongefish-dsfs

Added as the 5th workspace member. Contains the moved dsfs::* module from spongefish/src/dsfs/:

• compile.rs — Dsfs<IA, S, H, SALT_LEN> and DsfsReduction<IR, S, H, SALT_LEN> wrappers implementing NonInteractiveArgument / NonInteractiveReduction.
• channel.rs — SpongeProver, SpongeVerifier, TranscriptSponge.
• params.rs — DSFS-specific SpongeParams, SpongeInfo, STD_SPONGE_PARAMS, STD_HASH_SPONGE_PARAMS. Keccak and StdHash are re-aliased from spongefish::instantiations::*.
• narg_security.rs — NargSecurity, security_for_*, reduction_security_for_*.

Codec traits + foreign-type impls returned to spongefish

• Encoding, Decoding, Codec, ByteArray in spongefish/src/codecs.rs.
• NargSerialize, NargDeserialize in spongefish/src/io.rs.
• Deserialize in new spongefish/src/deserialize.rs.
• VerificationError, VerificationResult in spongefish/src/error.rs.
• Foreign-type codec impls (ark_ff field extensions, ark_ec curves, bls12_381, curve25519-dalek, p256, k256, p3 fields) merged into spongefish/src/drivers/<type>.rs alongside the existing Unit impls. SmallFp codec impls added on top of the existing SmallFp Unit impl from chore(deps): update actions/deploy-pages action to v5 arkworks-rs/spongefish#126.

pub(crate) flip on the free-fn surface

The 16 free fns (prove, verify, prove_with_salt, …, verify_reduction_with_sponge_and_salt) are now either pub(crate) (4 internal helpers used by the wrappers) or deleted (13 dead variants). The Dsfs::new(...).prove(...) / DsfsReduction::new(...).prove(...) wrappers are the only public DSFS surface.

Dsfs<IA, S, H = Keccak, const SALT_LEN: usize = 0> already covers every variant the deleted free fns expressed:

// custom salt length
Dsfs::<_, _, _, 16>::new(ia, Keccak::default())

// custom sponge
Dsfs::<_, _, StdHash, 0>::new(ia, StdHash::default())

Workspace plumbing

• [patch.crates-io] spongefish = { path = "spongefish" } added so transitive deps (ia-core reached through spongefish-dsfs) see the local copy instead of any stale crates.io publish.
• spongefish/Cargo.toml drops ia-core.workspace = true and the 9 "ia-core/<feat>" chain entries in [features]. Foreign-type features now gate spongefish's own dep: entries directly.

What does NOT change

• Transcript discipline: public inputs absorbed before first challenge, prover messages absorbed before next challenge, deterministic verifier replay, EOF check.
• DomainSeparator::derive derivation (length-prefixed SHA-512 over (protocol_id, sponge_info, session)).
• Sponge choices (Keccak and StdHash). TranscriptSponge impl for StdHash continues to use std_prover/std_verifier for byte-compat with σ-proofs Nizk.

Verification

cd ~/Developer/spongefish
cargo build --workspace --all-features
cargo test  --workspace --all-features      # all pass
cargo build -p spongefish --no-default-features
cargo tree  -p spongefish --invert -e normal | grep -c ia-core   # → 0

Local: all spongefish workspace tests pass. ia-core (in Argus) builds and tests pass against this branch via the path-dep override.

Coordination

This is one half of a coordinated refactor. The Argus-side companion lives at:

• ricardo-perello/argus:refactor/spongefish-dsfs-callsites — ia-core re-exports codec traits from spongefish; protocol crates pick up the spongefish-dsfs dep; call sites swept onto the wrapper.

The Argus PR depends on this one — merge spongefish-side first.

Base / stacking

Branched off dsfs/domain-separator-derive (PR arkworks-rs#114). Leaves that branch untouched. Carries 2 commits:

1. f201a55 refactor: extract DSFS into spongefish-dsfs sibling crate; codecs return to spongefish
2. 74fee6b refactor(dsfs): flip free fns to pub(crate); delete dead variants

Follow-ups (out of scope for this PR)

• Argus's dmpierre/ark-codes (used by warp) needs an ark-* 0.6 fork before warp can re-enter the workspace. Tracked Argus-side.
• σ-proofs golden vectors in sigma-bridge will need regeneration once 0010ea7 Require sessions in domain separator macro settles upstream — same canary as PR DomainSeparator::derive: mandatory (protocol_id, sponge_info, session) and remove new / .session() arkworks-rs/spongefish#114.

A couple of things you might want to tweak before posting:

• The "Coordination" section assumes the Argus PR is on ricardo-perello/argus; adjust if the canonical target is different.
• If you decide to split PR 1 (codec move) and PR 3 (pub(crate) flip) into separate PRs, just drop the "Pub(crate) flip" section and the 74fee6b commit reference.