feat: add Gamma hardfork E2E test with bridge verification

[ByteYue]

Add a comprehensive E2E test suite for the Gamma hardfork at
gravity_e2e/cluster_test_cases/hardfork_test/.

Hardfork Test (test_hardfork.py)

Boots a single-node devnet using v1.0.0 genesis contracts, then validates
the full Gamma hardfork lifecycle across 6 phases:

1. Pre-hardfork chain liveness
2. System contract bytecode snapshot + transaction pressure
3. Hardfork transition at configurable gammaBlock (default 500)
4. Post-hardfork bytecode verification — confirms all 10 system contracts
   are upgraded (StakingConfig, ValidatorConfig, GovernanceConfig, Staking,
   ValidatorManagement, Reconfiguration, Blocker, PerformanceTracker,
   Governance, NativeOracle)
5. Epoch boundary crossing — bytecodes remain stable
6. Node stop → restart → replay through hardfork block

Bridge Test (test_hardfork_bridge.py)

Verifies cross-chain bridge functionality across the hardfork boundary:

• Phase A: 10 pre-loaded bridge events (MockAnvil) → NativeMinted verified
• Phase B: Hardfork contract upgrade confirmation
• Phase C: 10 additional bridge events → verified post-hardfork with
  continuous nonces (1→20) and correct cumulative balance

Infrastructure

• hooks.py: Combined pre-start hook — starts MockAnvil on port 8546,
  injects gammaBlock into genesis.json, writes relayer_config.json
• genesis.toml: Uses gravity-testnet-v1.0.0 contracts with
  oracle+bridge config targeting local MockAnvil (chainId 31337)
• hardfork_utils.py: Reusable utilities for block waiting, contract
  snapshot/comparison, and transaction pressure