refactor(core): simplify JIT module by removing unused execution modes

Remove tree-walking interpreter and bytecode VM executor, keeping only:
- JIT mode (fnJIT): compiles to JavaScript via new Function()
- Exec mode (fnExec): closure-based execution (renamed from fnExecClosure)

Removed:
- fnExecTreeWalk, fnExecBytecode functions
- ContinueStmt, BreakStmt interfaces (unused)
- ~1200 lines of interpreter/VM code
- exec-modes.ts benchmark (compared removed modes)

The closure-based executor provides good performance for CSP-compliant
environments where new Function() is blocked, while being simpler to
maintain than the tree-walking interpreter or bytecode VM.

Author: marcj