Integrate Verifiable Random Function (VRF) for Block Proposer Selection

[Steake]

Part of Epic: #70 - RC1: Network & Consensus Foundation

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Goal: Use elliptic curve VRF to randomize block proposer selection for each round.

Requirements:

• Implement VRF output and proof system (schnorrkel or similar)
• Store and validate vrf_output and vrf_proof in BlockHeader
• Make block proposer selection nondeterministic and secure

Files:

• crates/bitcell-node/src/blockchain.rs
• crates/bitcell-crypto/src/vrf.rs

Acceptance Criteria:

• Block header contains real VRF data
• Block validation checks VRF proof
• Block proposer randomness is verified

See [AGENT_PLAN.md](https://github. com/Steake/BitCell/blob/master/AGENT_PLAN.md) and todo_now.md.

[Steake]

@copilot please work on this issue.

Dependencies completed:

• No dependencies

Please create a PR when ready.