hardening: add static assertion for WASM mavros stub type layout

Context

From PR #246 review: the WASM stub types for WitnessLayout and ConstraintsLayout in provekit/common/src/mavros.rs must exactly mirror the real mavros_vm types for postcard serialization compatibility. Currently enforced only by a comment.

If mavros_vm adds a field, the stubs silently diverge and deserialization fails with a confusing error instead of the clean "Mavros prover is not supported on WASM" message.

Proposed Fix

Add a compile-time size assertion on native targets:

#[cfg(not(target_arch = "wasm32"))]
const _: () = {
    assert!(std::mem::size_of::<mavros_vm::WitnessLayout>() == 5 * std::mem::size_of::<usize>());
    assert!(std::mem::size_of::<mavros_vm::ConstraintsLayout>() == 3 * std::mem::size_of::<usize>());
};

Or a test that roundtrips both real and stub types through postcard to verify wire compatibility.

Start in provekit/common/src/mavros.rs, where the WASM stub types for WitnessLayout and ConstraintsLayout are defined, and compare them with the native mavros_vm types. Add the requested native-target compile-time layout checks or a postcard roundtrip test; done means layout divergence is caught clearly while WASM remains unsupported.