refactor: MIR builder - complete modularization (Phase 6-8)
- Phase 6: statements.rs (165 lines) - 6 functions
- build_print_statement, build_block
- build_local_statement, build_return_statement
- build_throw_statement, build_nowait_statement
- Phase 7: control_flow.rs (194 lines) - 4 functions
- build_if_statement, build_loop_statement
- build_try_catch_statement, is_current_block_terminated
- Phase 8: box_handlers.rs (73 lines) - 2 functions
- build_static_main_box, build_box_declaration
Total: 30 functions moved across 4 modules
Original builder.rs reduced from 1547 lines
Build verified successfully
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