node: move proposal signing into validator client

[zclawz]

Summary

Implements the TODO in pkgs/node/src/chain.zig around proposal signing/proof construction by moving validator-owned work out of chain.zig and into validator_client.zig.

Changes:

• Removes the TODO block from BeamChain.proposeImpl.
• Adds ValidatorClient.buildProposalOutput, which:
  • signs the produced block root with the validator key manager,
  • builds the Type-2 block proof via the chain proof helper,
  • folds/free Type-1 attestation proofs,
  • returns the signed block as a validator gossip output (GossipMessage.block).
• Adds a small pending validator-output queue on BeamNode so off-loop proposal workers can hand signed block output back to the interval tick.
• Reuses the node's existing validator output publish path so BeamNode remains the publisher for blocks/attestations/aggregations.

Tracking topic: https://t.me/zeamETH/7164

Validation

Running:

• zig fmt --check pkgs/node/src/chain.zig pkgs/node/src/node.zig pkgs/node/src/validator_client.zig
• zig build test --summary all --color off

I will update this PR comment when the full test run completes.

[zclawz]

Validation complete ✅

• zig fmt --check pkgs/node/src/chain.zig pkgs/node/src/node.zig pkgs/node/src/validator_client.zig passed
• zig build test --summary all --color off passed (EXIT:0)

Log: /tmp/pr-move-validator-output-test.log