feat: add proposer proof aggregation (#284)

This PR adds true attestation aggregation on the proposer block building
pipeline. As part of this, the attestation byte cap we added was removed
since we now have the 16 attestation+proof cap.

Author: MegaRedHand

CLAUDE.md

@@ -338,10 +338,9 @@ cargo test -p ethlambda-blockchain --test forkchoice_spectests -- --test-threads
 ## Common Gotchas
 
 ### Aggregator Flag Required for Finalization
-- At least one node **must** be started with `--is-aggregator` to finalize blocks in production (without `skip-signature-verification`)
+- At least one node **must** be started with `--is-aggregator` to finalize blocks
 - Without this flag, attestations pass signature verification and are logged as "Attestation processed", but the signature is never stored for aggregation (`store.rs:368`), so blocks are always built with `attestation_count=0`
 - The attestation pipeline: gossip → verify signature → store gossip signature (only if `is_aggregator`) → aggregate at interval 2 → promote to known → pack into blocks
-- With `skip-signature-verification` (tests only), attestations bypass aggregation and go directly to `new_aggregated_payloads`, so the flag is not needed
 - **Symptom**: `justified_slot=0` and `finalized_slot=0` indefinitely despite healthy block production and attestation gossip
 
 ### Signature Verification