refactor(interpreter): extract inline builtins from execute_dispatched_command (#485)

## Summary
- Extracted 6 inline command handlers from the 574-line
`execute_dispatched_command()` into separate methods
- New methods: `execute_function_call`, `execute_local_builtin`,
`execute_trap_builtin`, `execute_let_builtin`, `execute_unset_builtin`,
`execute_caller_builtin`
- No behavioral changes, pure refactoring

Closes #313

## Test plan
- [x] All existing tests pass (no behavioral changes)
- [x] clippy clean

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

Author: chaliy