make fx_hashmap_serde public for downstream reuse

[flyq]

Summary

• Promote the fx_hashmap_serde helper module in salt/src/proof/prover.rs from private to pub, and re-export it from salt::proof and the crate root.
• Convert the explanatory block comment into a doc comment and note that downstream crates (e.g. stateless-core::LightWitness) can now reuse the helper via #[serde(with = "salt::fx_hashmap_serde")].

Why

The hand-written serde helper exists so salt doesn't have to enable hashbrown/serde (which transitively pulls in serde_core 1.0.221+ and breaks alloy-tx-macros 1.0.23). Downstream crates hit the same constraint when serializing FxHashMap fields, so re-exporting the helper lets them apply the same workaround instead of reinventing it or re-introducing the broken transitive dep.

Test plan

• cargo build -p salt
• cargo test -p salt
• Verify a downstream consumer can reference salt::fx_hashmap_serde in #[serde(with = ...)]

[github-actions]

Performance Benchmark Comparison

Compared 5 benchmark(s) against the latest main baseline.

Detailed Comparison

Benchmark	Baseline Throughput (Kelem/s)	New Throughput (Kelem/s)	Change
update 10000 KVs/1 threads	43.52	42.71	-1.85%
update 10000 KVs/2 threads	80.63	80.21	-0.52%
update 10000 KVs/4 threads	149.17	147.63	-1.03%
update 10000 KVs/8 threads	262.18	260.84	-0.51%
update 10000 KVs/16 threads	391.49	390.86	-0.16%