fix(serdes): decode usize as u64 on 32-bit targets (wasm32 verifier)

[pauldelucia]

Problem

exp_serde_for_number!(usize, 8) expands to a usize::from_le_bytes([u8; 8]) call. On 32-bit targets (wasm32-unknown-unknown, wasm32-wasi) usize::from_le_bytes takes [u8; 4], so the macro fails to compile.

Fix

Replace the macro call for usize with a hand-written ExpSerde impl that:

• Serializes as a fixed-width 8 bytes (matches current 64-bit behavior — wire format unchanged).
• Deserializes by reading 8 bytes into a u64, then try_from-ing to usize. Values above usize::MAX on 32-bit (4 GiB) return SerdeError::DeserializeError.

Why

Proofs are produced on 64-bit hosts; the wire format already stores usize as 8 bytes. This is purely a 32-bit-decode fix — no change to what 64-bit consumers see.

Motivation: unblocks browser / mobile WASM verifier builds that need to deserialize Circuit<C> and Proof on wasm32-unknown-unknown.

Scope

• 18 lines in serdes/src/serdes.rs.
• No wire-format change.
• No test fixtures need regenerating.

[gemini-code-assist]

Code Review

This pull request replaces the macro-based serialization for usize with a manual implementation to ensure it always serializes as a fixed-width 64-bit integer, which enables cross-platform compatibility between 64-bit provers and 32-bit WASM verifiers. A review comment suggests simplifying this manual implementation by delegating to the existing u64 serialization logic to improve idiomaticity and reduce code duplication.

[gemini-code-assist]

The manual implementation of ExpSerde for usize can be simplified by delegating to the u64 implementation. This is more idiomatic, reduces code duplication, and matches the pattern used for the bool implementation later in this file. It correctly preserves the 8-byte wire format and the try_from check for 32-bit targets.

    fn serialize_into<W: Write>(&self, writer: W) -> SerdeResult<()> {
        (*self as u64).serialize_into(writer)
    }
    fn deserialize_from<R: Read>(reader: R) -> SerdeResult<Self> {
        let v = u64::deserialize_from(reader)?;
        usize::try_from(v).map_err(|_| SerdeError::DeserializeError)
    }

[pauldelucia]

Good call — delegated to u64::ExpSerde in 6a9e39e.