Implement Zero-Knowledge Proof (ZKP) Circuits for Consensus Verification

[Steake]

Part of Epic: #71 - RC1: Zero-Knowledge & Observability

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Goal: Replace stubbed ZKP code with real Groth16-based circuits for battle outcome and state transitions.

Requirements:

• Implement Groth16Proof struct with real proof verification
• Real constraints for CA evolution and state transition
• Update BattleCircuit, StateCircuit code for proof generation and verification

Files:

• crates/bitcell-zkp/src/lib.rs
• crates/bitcell-zkp/src/battle_circuit.rs
• crates/bitcell-zkp/src/state_circuit.rs

Acceptance Criteria:

• Valid battles and transitions result in cryptographically verified proofs
• No reliance on stub checks for consensus

See [todo_now.md](https://github.com/Steake/BitCell/blob/master/todo_now. md) for audit findings.

[Steake]

[autopilot] @copilot please start work on this issue and open a PR that references #40 in the PR description.