feat(validator): return signed block commitment from SignBlock

[mmagician]

Why

Block 1 is being rejected by the block producer with InvalidSignature
(signature.verify(header.commitment(), header.validator_key())). #2203 rules out the validator-key mismatch. The remaining unverified suspect is that the validator and the block producer derive a different block commitment from the same ProposedBlock (e.g. an architecture-dependent difference - the validator runs on ARM, dev/CI on amd64), so the signature is valid for the validator's commitment but not the producer's.

Today the SignBlock response carries only the signature, so the producer cannot tell a commitment mismatch from a key/algorithm problem. This PR has the validator return the commitment it actually signed, and the block producer compares it against its own. It is a deliberately small, easily revertible diagnostic, separate from #2203.

What

• Proto: new SignBlockResponse { BlockSignature signature; primitives.Digest block_commitment; }, and SignBlock now returns it. BlockSignature is left unchanged (it is also embedded in the persisted SignedBlock, where the commitment would be redundant with the header), so the existing conversions are untouched.
• Validator handler (bin/validator/src/server/sign_block.rs): returns (Signature, Word); the signed commitment is captured from the validated header before it is moved into the persistence step.
• Block producer client (crates/block-producer/src/validator/mod.rs): sign_block returns (Signature, Word).
• Block producer (crates/block-producer/src/block_builder/mod.rs): before the signature check, compares the validator's signed commitment against the locally built header.commitment() and returns the new BuildBlockError::BlockCommitmentMismatch { validator, block_producer } on divergence. This cleanly separates the cases: commitment mismatch -> BlockCommitmentMismatch; commitment matches but verify fails -> key/algorithm problem (InvalidSignature).

A hard error (not assert!/panic) is used, matching the existing InvalidSignature style: on a real mismatch the signature check would fail anyway, so the explicit error is strictly more informative without crashing the process.

Tests

sign_block_returns_signed_commitment (validator server suite) asserts the response carries the commitment of the block the validator signed and that it matches the commitment derived from the same proposed block.

Verification

• cargo build -p miden-node-proto -p miden-validator -p miden-node-block-producer (regenerates proto).
• cargo test -p miden-validator --lib: 14 passed, 1 ignored.
• cargo clippy --locked -p miden-node-proto -p miden-validator -p miden-node-block-producer --all-targets --all-features -- -D warnings: clean.
• Nightly rustfmt + comment reflow: clean.

(Note: mempool::tests::add_transaction_traces_are_correct is flaky under parallel runs - passes in isolation - and is unrelated to this change.)

🤖 Generated with Claude Code

[bobbinth]

Looks good! Thank you! (I reviewed non-test code)