Streamline transaction execution and prepared bytecode reuse

[0xjojoex]

Reduce transaction-shell and repeated code-preparation overhead while making transaction lifetime explicit.

• Change Vm.transact to return a journal-backed PendingTransaction.
  • accept() retains the transaction in the overlay.
  • reject() restores the pre-transaction state without allocation.
  • unresolved transactions reject automatically on deinit.
  • transactCommit remains the accept-and-persist convenience path.
• Skip full overlay snapshots when no before-transaction hooks exist.
• Prepare all bytecode before execution; the interpreter now consumes immutable Bytecode.
• Add an optional caller-owned prepared_code_backend for cross-transaction reuse.
  • ship InMemoryPreparedPool as the default implementation;
  • keep misses and backend-less execution transaction-scoped;
  • pin prepared artifacts across root and nested frames.
• Reuse resolved code across calls and skip empty sender-code reads on the EOA path.
• Add calldata zero-count and ModExp fast paths, including differential fuzz coverage.

ExecutionConfig.Preprocessing.none is removed. Execution always uses prepared bytecode; caching remains optional.