Fix 17 audit findings: H-01 action_type binding, H-02 double-resolution, H-03 Merkle verification, M-01/M-02 persistent storage, M-03 max_witnesses, M-04 deployment warnings, M-05 u64 arithmetic, M-06 positive amount checks, L-01-L-08 minor fixes

security: checks-effects-interactions fix in fleet_learning claim_reward

Add fleet_learning: ZK-verified FedAvg aggregation coordinator + autonomous per-contributor Stellar rewards

Add fleet_learning unit tests: submission gating, Poseidon root fold, claim gating, load-bearing proof check