refactor: make Context and Evm generic over DB type (#20)

* feat(eip7928): BAL-aware access tracking infrastructure

Add the journal/database plumbing required for correct EIP-7928 Block
Access List construction:

- primitives: fix TX_GAS_LIMIT_CAP to 16777216 (EIP-7825 value)
- database: FallbackFns vtable gains commit_tx/discard_tx/snapshot_frame/
  commit_frame/revert_frame/untrack_address hooks so a stateless witness
  DB can track accesses with frame-accurate reverts
- context/journal: lazy access-list pre-warming (warmAccessList / setAccessList)
  replaces eager loadAccountWithCode+sload; adds isAddressCold/isStorageCold
  predicates; StorageChanged journal entry now records old_was_written for
  correct revert; snapshotFrame/commitFrame/revertFrame forwarded to db
- handler/mainnet_builder: use warmAccessList for EIP-2929 pre-warming;
  call snapshotFrame/revertFrame around top-level CALL value transfer
- interpreter/host: propagate snapshotFrame/commitFrame/revertFrame through
  CALL/CREATE frame lifecycle
- interpreter/opcodes/call + host_ops: use isAddressCold/isStorageCold for
  pre-call gas checks without triggering DB loads; untrack_address on OOG
- state: storage slot gains was_written flag for revert correctness

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* Remove debug prints; fix EIP-3651 coinbase cross-tx warm check

- Remove DBG prints from InMemoryDB.basic and JournalInner.setCodeWithHash
- Fix isAddressCold: only treat coinbase as cross-tx warm (EIP-3651),
  not all previously-loaded addresses, to avoid unintended gas discounts
  for precompiles and access-list entries

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* feat(bal): fix EIP-7928 BAL tracking for 7702 delegation OOG boundaries

Split the CALL instruction OOG check into two stages to match EELS:
1. If remaining < call_cost_no_delegation: untrack target (oog_before_target_access)
2. If remaining < base_cost (includes delegation): halt OOG without untracking target
   (oog_after_target_access and oog_success_minus_1 — target IS in BAL, delegation NOT)

Also add unconditional untrackAddress for OOG in EXTCODECOPY and CALL-type
opcodes when gas is exhausted before the target is accessed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* style: apply zig fmt

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* refactor: make Context generic over DB, remove FallbackFns vtable

- Remove FallbackFns C-style vtable and InMemoryDB.fallback field
- Make Context generic: Context(comptime DB: type); DefaultContext = Context(InMemoryDB)
- Add Journal(DB) tracking wrappers guarded by @hasDecl (snapshotFrame, commitFrame,
  revertFrame, commitTracking, discardTracking, notifyStorageSlotCommit,
  hasNonZeroStorageForAddress, untrackAddress, forceTrackAddress)
- Replace Host.ctx with type-erased JournalVTable (18 entries); Host.init(DB, ctx, prec)
  and Host.fromCtx(ctx, prec) constructors; opcode handlers unchanged
- Make Evm generic: EvmFor(comptime DB: type); Evm = EvmFor(InMemoryDB) alias
- Make MainnetHandler.* functions accept anytype evm for zero-cost duck typing
- Fix getDb() dangling pointer: change self: @this() → self: *const @this()
- Update test files to use Host.fromCtx() instead of Host{ .ctx = ... } literal

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* perf/refactor: O(1) CREATE storage collision check; simplify Frame.execute

- Add nonzero_storage_count index to InMemoryDB maintained by new private
  putStorage helper; hasNonZeroStorageForAddress is now O(1) instead of
  O(n) storage scan on every CREATE
- Drop explicit `comptime DB: type` parameter from Frame.execute; infer DB
  via anytype ctx to remove parameter sprawl

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix: propagate spec in withCfg/modifyCfgChained; add revertFrame on transfer failure

- withCfg and modifyCfgChained now call setSpecId on the returned
  context's journaled_state so the journal spec stays in sync
- setupCallCore now calls revertFrame() before checkpointRevert() on
  the two early-return paths when value transfer fails, matching the
  pattern in mainnet_builder.zig

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Gabriel-Trintinalia

examples/simple_contract.zig

@@ -18,7 +18,7 @@ pub fn main() !void {
     defer db.deinit();
 
     // Create a context with Prague specification
-    var ctx = context.Context.new(db, primitives.SpecId.prague);
+    var ctx = context.DefaultContext.new(db, primitives.SpecId.prague);
 
     // Create a simple contract that adds two numbers
     // PUSH1 0x05  (push 5 onto stack)