…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

Per EIP-7928, coinbase must appear in BAL when there's a user transaction,
even if the priority fee is zero. System contract calls have gas_price = 0,
so we use this to distinguish them from user transactions.

Block 2753572 (first post-Transition) committed cleanly under Bug 4 fix.
Block 2753573 fails with off-by-3 nonce mismatch — different magnitude
than prior rounds, suggesting either incomplete Bug 3 v3 coverage or a
problem in BinaryBackend::update_accounts intra-block state propagation.
Documents diagnostic ideas for next session.

Adds [BINARY-DEBUG]-tagged logs at four points to triangulate the off-by-N
nonce mismatch on hoodi sign-off run #5:

1. levm default_hook: prints sender address + state vs tx nonce on
   NonceMismatch (eprintln! since levm has no tracing).
2. binary-trie backend update_accounts: prints (addr, nonce, balance,
   code_hash) for every binary account write (eprintln! same reason).
3. storage store apply_trie_updates: tracing::info on each
   current_binary_root advance (parent + child roots).
4. storage force_commit_layers: tracing::info summary at the end with
   total nodes + leaves committed during activation freeze.

All four are TEMP — remove once the residual is fixed on a clean hoodi
sign-off.