feat(mir): Phase 32 Step 0-3B - LoopRegion/LoopControlShape view methods

Phase 32 introduces a "view pattern" for loop structures, enabling
gradual migration without breaking existing code.

Changes:
- control_form.rs: Add new ID types (LoopId, ExitEdgeId, ContinueEdgeId)
  and structures (LoopRegion, LoopControlShape, ExitEdge, ContinueEdge)
- control_form.rs: Add view methods on LoopShape:
  - to_region_view() - returns LoopRegion
  - to_control_view() - returns LoopControlShape
  - to_exit_edges() - returns Vec<ExitEdge>
  - to_continue_edges() - returns Vec<ContinueEdge>
- loop_scope_shape.rs: Use views in from_existing_boxes_legacy()
- loop_to_join.rs: Add debug logging with Phase 32 views

All 4 minimal lowerers (skip_ws/trim/append_defs/stage1) now use
view-based block ID extraction via shared from_existing_boxes_legacy().

Tests: joinir_runner_standalone_*, joinir_vm_bridge_trim_* PASS

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

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

src/mir/join_ir/lowering/loop_to_join.rs

@@ -15,6 +15,7 @@
 //! let join_module = lowerer.lower(func, &loop_form, &query)?;
 //! ```
 
+use crate::mir::control_form::LoopId;
 use crate::mir::join_ir::lowering::generic_case_a;
 use crate::mir::join_ir::lowering::loop_form_intake::intake_loop_form;
 use crate::mir::join_ir::lowering::loop_scope_shape::LoopScopeShape;
@@ -108,6 +109,23 @@ impl LoopToJoinLowerer {
             );
         }
 
+        // Phase 32 Step 3-A: View メソッドを使った構造確認（段階移行）
+        if self.debug {
+            let loop_id = LoopId(0); // 単一ループの場合は 0
+            let region = loop_form.to_region_view(loop_id);
+            let control = loop_form.to_control_view(loop_id);
+            let exit_edges = loop_form.to_exit_edges(loop_id);
+            let continue_edges = loop_form.to_continue_edges(loop_id);
+
+            eprintln!(
+                "[LoopToJoinLowerer] Phase 32 views: region.header={:?}, control.exits={}, exit_edges={}, continue_edges={}",
+                region.header,
+                control.exits.len(),
+                exit_edges.len(),
+                continue_edges.len()
+            );
+        }
+
         // Step 5: パターンに応じた lowering を実行
         self.lower_with_scope(scope, func_name)
     }