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>

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)