feat(joinir): Phase 40-4.1 delete collect_assigned_vars (35 lines)

Breaking: collect_assigned_vars function removed from if_phi.rs

Changes:
- Delete collect_assigned_vars() function (35 lines)
- Make JoinIR route the default in loop_builder.rs
- Rewrite collect_assigned_vars_via_joinir() with ast_to_json support
  - Now detects both Local declarations and Assignment nodes
  - Add extract_vars_from_json_stmts/stmt helpers
- Update tests to use new implementation
  - phase40_joinir_detects_local_declarations
  - phase40_joinir_nested_if_local

Test results: 407 passed, 11 failed (same as before, no regression)

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

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

src/mir/loop_builder.rs

@@ -1061,31 +1061,12 @@ impl<'a> LoopBuilder<'a> {
             .debug_push_region(format!("join#{}", join_id) + "/join");
 
         // Phase 25.1: HashSet → BTreeSet（決定性確保）
-        // Phase 40-3.5: JoinIR route switching
+        // Phase 40-4.1: JoinIR経路をデフォルト化（collect_assigned_vars削除）
         let vars: std::collections::BTreeSet<String> =
-            if crate::config::env::use_joinir_for_array_filter() {
-                // Route B: JoinIR Frontend経由
-                crate::mir::phi_core::if_phi::collect_assigned_vars_via_joinir(
-                    &then_body,
-                    else_body.as_ref(),
-                )
-            } else {
-                // Route A: Legacy AST→MIR経由
-                let mut vars = std::collections::BTreeSet::new();
-                let then_prog = ASTNode::Program {
-                    statements: then_body.clone(),
-                    span: crate::ast::Span::unknown(),
-                };
-                crate::mir::phi_core::if_phi::collect_assigned_vars(&then_prog, &mut vars);
-                if let Some(es) = &else_body {
-                    let else_prog = ASTNode::Program {
-                        statements: es.clone(),
-                        span: crate::ast::Span::unknown(),
-                    };
-                    crate::mir::phi_core::if_phi::collect_assigned_vars(&else_prog, &mut vars);
-                }
-                vars
-            };
+            crate::mir::phi_core::if_phi::collect_assigned_vars_via_joinir(
+                &then_body,
+                else_body.as_ref(),
+            );
 
         // Phase 26-E: PhiBuilderBox 統合
         // Ops構造体: PhiMergeOps（Legacy）と PhiBuilderOps（新）の両対応