feat(joinir): Phase 34-7.5 helpers + Phase 34-8 Break/Continue implementation

Phase 34-7.5: Code organization improvements
- Added type conversion helpers (as_cont/as_func) in join_ir/mod.rs
- Enhanced docstrings for JoinCall/JoinJump with usage examples
- Improved error messages in join_ir_vm_bridge.rs

JoinIrFrontendTestRunner box implementation
- Created src/tests/helpers/joinir_frontend.rs (119 lines)
- Reduced test code by 83% (70 lines → 12 lines per test)
- 26% overall reduction in test file (284 → 209 lines)

Phase 34-8: Break/Continue pattern implementation
- Extended ast_lowerer.rs (+630 lines)
  - lower_loop_break_pattern(): Break as Jump (early return)
  - lower_loop_continue_pattern(): Continue as Select + Call
  - Added Bool literal support in extract_value()
- Created 2 fixtures: loop_frontend_{break,continue}.program.json
- Added 2 A/B tests (all 6 Phase 34 tests PASS)

Technical achievements:
- Break = Jump (early return pattern)
- Continue = Select + Call (NOT Jump) - critical discovery
- 3-function structure sufficient (no k_continue needed)
- SSA-style re-assignment with natural var_map updates

Test results:
✅ joinir_frontend_if_select_simple_ab_test
✅ joinir_frontend_if_select_local_ab_test
✅ joinir_frontend_json_shape_read_value_ab_test
✅ joinir_frontend_loop_simple_ab_test
✅ joinir_frontend_loop_break_ab_test
✅ joinir_frontend_loop_continue_ab_test

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/tests/mod.rs

@@ -1,3 +1,5 @@
+mod helpers;
+
 #[cfg(feature = "aot-plan-import")]
 pub mod aot_plan_import;
 pub mod box_tests;