feat(joinir): Phase 40-3.5 route switching implementation

- Add collect_assigned_vars_via_joinir() in if_phi.rs (65 lines)
  - Wrapper using Phase 40-1 JoinIR infrastructure
  - Converts ASTNode to JSON and calls JoinIR analysis
- Add route switching in loop_builder.rs
  - Check HAKO_JOINIR_ARRAY_FILTER env flag
  - Route A: Legacy collect_assigned_vars path
  - Route B: JoinIR collect_assigned_vars_via_joinir path
- Add A/B tests in phase40_array_ext_filter_test.rs
  - phase40_ab_route_switching: Basic assignment detection
  - phase40_ab_nested_if: Nested if assignment detection
- All 5 Phase 40 tests PASS

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

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

src/mir/phi_core/if_phi.rs

@@ -161,6 +161,71 @@ pub fn collect_assigned_vars(ast: &ASTNode, out: &mut std::collections::BTreeSet
     }
 }
 
+/// Phase 40-3.5: JoinIR経由の代入変数収集
+///
+/// `collect_assigned_vars`のJoinIR代替版。
+/// `HAKO_JOINIR_ARRAY_FILTER=1`で有効化。
+///
+/// # Arguments
+/// - `then_body`: thenブランチのAST
+/// - `else_body`: elseブランチのAST（オプション）
+///
+/// # Returns
+/// - 代入された変数名のセット
+///
+/// # Implementation
+/// 1. ASTNode → JSON変換
+/// 2. JoinIR Frontend extract_assigned_vars_from_body()呼び出し
+/// 3. BTreeSet形式で返却
+///
+/// # Note
+/// この関数はPhase 40-1インフラを使用し、将来的にcollect_assigned_varsを置換する。
+#[allow(dead_code)]
+pub fn collect_assigned_vars_via_joinir(
+    then_body: &[crate::ast::ASTNode],
+    else_body: Option<&Vec<crate::ast::ASTNode>>,
+) -> std::collections::BTreeSet<String> {
+    use crate::mir::join_ir::frontend::AstToJoinIrLowerer;
+
+    let mut result = std::collections::BTreeSet::new();
+
+    // Convert then_body to JSON
+    let then_prog = crate::ast::ASTNode::Program {
+        statements: then_body.to_vec(),
+        span: crate::ast::Span::unknown(),
+    };
+    let then_json = crate::r#macro::ast_json::ast_to_json(&then_prog);
+
+    // Extract assigned vars via JoinIR
+    let mut lowerer = AstToJoinIrLowerer::new();
+    if let Some(body) = then_json.get("statements") {
+        let assigned = lowerer.extract_assigned_vars_from_body(body);
+        result.extend(assigned);
+    }
+
+    // Process else_body if present
+    if let Some(else_statements) = else_body {
+        let else_prog = crate::ast::ASTNode::Program {
+            statements: else_statements.clone(),
+            span: crate::ast::Span::unknown(),
+        };
+        let else_json = crate::r#macro::ast_json::ast_to_json(&else_prog);
+        if let Some(body) = else_json.get("statements") {
+            let assigned = lowerer.extract_assigned_vars_from_body(body);
+            result.extend(assigned);
+        }
+    }
+
+    if crate::config::env::joinir_vm_bridge_debug() {
+        eprintln!(
+            "[Phase 40-3.5] collect_assigned_vars_via_joinir: {:?}",
+            result
+        );
+    }
+
+    result
+}
+
 /// 保守的修正名前計算
 ///
 /// Compute the set of variable names whose values changed in either branch