refactor: split Prove trait into target-stable base + native-only extension

[Bisht13]

Context

From PR #246 review (@ashpect):

The Prove trait currently has cfg-gated methods that change the API surface between native and WASM targets:

pub trait Prove {
    #[cfg(all(feature = "witness-generation", not(target_arch = "wasm32")))]
    fn prove(self, input_map: InputMap) -> Result<NoirProof>;

    #[cfg(all(feature = "witness-generation", not(target_arch = "wasm32")))]
    fn prove_with_toml(self, prover_toml: impl AsRef<Path>) -> Result<NoirProof>;

    fn prove_with_witness(self, witness: WitnessMap<NoirElement>) -> Result<NoirProof>;
}

Generic code written on native that calls .prove() will fail to compile on WASM with a confusing "method not found" error.

Proposed Change

Split into a stable base trait and a native-only extension:

/// Always available (including WASM)
pub trait Prove {
    fn prove_with_witness(self, witness: WitnessMap<NoirElement>) -> Result<NoirProof>;
}

/// Native-only extension
#[cfg(not(target_arch = "wasm32"))]
pub trait ProveFromInput: Prove {
    fn prove(self, input_map: InputMap) -> Result<NoirProof>;
    fn prove_with_toml(self, prover_toml: impl AsRef<Path>) -> Result<NoirProof>;
}

Also noted: the double-gating #[cfg(all(feature = "witness-generation", not(target_arch = "wasm32")))] is redundant — the WASM Cargo.toml already uses default-features = false which excludes witness-generation. The feature flag alone should suffice.

Impact

• All callers of .prove() and .prove_with_toml() would need to import ProveFromInput instead of Prove
• No behavioral change, purely a type-system improvement