feat(joinir): Phase 33-9.1 Loop/If lowering responsibility separation

Implement clear separation between Loop and If lowering responsibilities:

**Guard Implementation:**
- Add is_loop_lowered_function() to identify 6 Loop-dedicated functions
- Exclude Loop functions from If lowering path in try_lower_if_to_joinir()
- Enforce "1 function → 1 lowering" principle

**Documentation:**
- Add responsibility comments to loop_to_join.rs
- Add responsibility comments to if_select.rs and if_merge.rs
- Update if_joinir_design.md with Phase 33-9.1 section

**Testing:**
- Add unit test test_is_loop_lowered_function() (PASS)
- Verify no regression in existing JoinIR tests

**Loop-dedicated functions (6):**
- Main.skip/1
- FuncScannerBox.trim/1
- FuncScannerBox.append_defs/2
- Stage1UsingResolverBox.resolve_for_source/5
- StageBBodyExtractorBox.build_body_src/2
- StageBFuncScannerBox.scan_all_boxes/1

This prevents future conflicts when both Loop and If lowering expand.

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

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

src/mir/join_ir/lowering/if_select.rs

@@ -1,6 +1,14 @@
 //! Phase 33: If/Else の Select 命令への lowering
 //!
 //! 最小の if/else（副作用なし、単純な値選択）を JoinInst::Select に変換する。
+//!
+//! ## 責務分離（Phase 33-9.1）
+//!
+//! **IfSelectLowerer の責務**:
+//! - 単純 if/else（副作用なし、単一変数）を Select 命令に変換する
+//!
+//! **非責務**:
+//! - Loop の PHI には触らない（Loop lowering の責務）
 
 use crate::mir::join_ir::JoinInst;
 use crate::mir::{BasicBlockId, MirFunction, MirInstruction, ValueId};