Harmonize the recursive and non-recursive SNARK implementations in STM

[jpraynaud]

Why

The recursive (IVC) and non-recursive SNARK stacks were built incrementally and drifted apart. Naming, module layout and preimage types differ for concepts that are the same, which makes the library harder to read, to review and to extend.

What

Align the two stacks on a single set of names, a single module structure and a single preimage representation, without changing the proving or verifying behavior.

How

• Harmonize IvcTransitionType and the IVC state transitions (proof_system/ivc_halo2_snark/prover_input_helpers.rs, rolling_state.rs)
• Harmonize the circuit namings IvcCircuitData (circuits/halo2_ivc/circuit.rs) and StmCertificateCircuit (circuits/halo2/circuit.rs)
• Align the module structure and the naming of the two proof systems (circuits/halo2 vs circuits/halo2_ivc, proof_system/halo2_snark vs proof_system/ivc_halo2_snark: prefix vs suffix, mod vs pub(crate) mod)
• Refactor ProtocolMessagePreimage (circuits/halo2_ivc/types.rs) and GenesisMessagePreimage (protocol/aggregate_signature/preimage.rs) into a consistent representation
• Remove the #[allow(dead_code)] markers left on IvcCircuitData once the recursive keys are wired