feat: batch kernel skeleton

[mmagician]

Summary

First PR in a two-part split of #2884 (kept as draft for reference).

Establishes the public input/output contract for the batch kernel (#1122) plus the Rust plumbing that surrounds it. The kernel does not yet perform any verification — it drops its public inputs and emits the empty word output shape. The verification chain (advice-map unhashing, per-tx header check, note-commitment computation) lands in PR 2 on top of this one.

• main.masm drops TRANSACTIONS_COMMITMENT + BLOCK_HASH and exits; the VM's depth ≥ 16 invariant leaves the all-zero 16-felt output region in place.
• BatchKernel Rust wrapper exposes prepare_inputs / build_input_stack / build_output_stack / parse_output_stack; build_advice_inputs returns the default empty AdviceInputs since the skeleton ignores advice data.
• ProvenBatch carries a proof: ExecutionProof field through new_unchecked and serde.
• LocalBatchProver::prove now runs the kernel via miden_prover::prove and attaches the proof to the returned ProvenBatch. The kernel's public outputs are not cross-checked against the proposed batch yet; that check lands with the verification logic.
• prove_dummy retained for tests that don't want proof generation.

The smoke test builds a realistic two-transaction ProposedBatch through MockChain + MockProvenTxBuilder, runs the kernel via FastProcessor, and asserts the parsed outputs are all empty / zero. It uses FastProcessor directly rather than LocalBatchProver::prove because the mock transactions carry dummy proofs that TransactionVerifier would reject.

The TODO list at the top of main.masm enumerates the checks PR 2 will introduce.

Test plan

• cargo nextest run -p miden-testing --lib kernel_tests::batch::batch_kernel
• make lint
• scripts/check-features.sh