feat(types): split validators pubkey into dual-key Validator

[dimka90]

Summary
This PR

• Split Validator.Pubkey into AttestationPubkey + ProposalPubkey (SSZ 60→112 bytes) for devnet-4 dual-key support. Both keys populated from same genesis key; Phase 2 will parse
  separate keys.
• Add MaxAttestationsData = 16 constant (PR #536).
• drainPendingBlocks before each tick — mirrors zeam's processPendingBlocks() to prevent Go select race between block arrival and tick processing.
• GossipSignatures dedup in InsertWithHandle — prevents duplicate entries when same attestation arrives from multiple gossip paths.
• PruneStaleSigs — bounds gossip signature growth on non-aggregator nodes (10-slot window).
• Makefile: don't delete *_encoding.go on make clean.

Changes

Types

• types/validator.go: Pubkey → AttestationPubkey + ProposalPubkey
• types/validator_encoding.go: SSZ encoding updated (60→112 bytes)
• types/state_encoding.go: Validator size references updated
• types/constants.go: Added MaxAttestationsData = 16

Node

• node/tick.go: Added drainPendingBlocks() — processes queued blocks before each tick to ensure head is current before attestation production
• node/store_gossip.go: Added dedup in InsertWithHandle, added PruneStaleSigs method
• node/store_block.go, node/store_aggregate.go, node/block.go, node/verify.go: .Pubkey → .AttestationPubkey

Genesis

• genesis/config.go: Populates both keys from same genesis key

Build

• Makefile: Removed rm -f types/*_encoding.go from clean target

Test plan

• All unit tests pass
• 3-node gean-only devnet: justification/finalization advancing steadily (justified within 5 slots of head)
• Verified main branch baseline still works
• Each improvement tested individually — no regressions