…rks (#5)

Implement fork-specific gas cost handling for the SSTORE opcode across
pre-merge Ethereum forks. This completes the core gas cost implementation
for pre-merge fork support.

Changes to gas_cost.rs:
- Add pre-Istanbul SSTORE constants: SSTORE_GAS_SET (20000 for zero to
  non-zero), SSTORE_GAS_RESET (5000 for other changes), and
  SSTORE_REFUND_CLEAR (15000 for clearing slots)
- Modify sstore() to accept fork parameter and use simple gas model for
  pre-Istanbul forks (no net gas metering)
- Apply EIP-2929 cold/warm access costs only for Berlin and later

Changes to stack_memory_storage_flow.rs:
- Pass fork to gas_cost::sstore() function
- Apply EIP-2200 stipend check only for Istanbul and later forks
- Implement pre-Istanbul refund logic: 15000 gas refund for clearing
  storage (non-zero to zero)
- Maintain EIP-2200 net gas metering refunds for Istanbul+ forks

Gas cost history for SSTORE:
- Frontier through Spurious Dragon: 20000 (set), 5000 (reset), 15000 refund
- Istanbul: EIP-2200 net gas metering
- Berlin+: EIP-2929 cold/warm access costs added