chore(joinir): Phase 40-4.1.1 dead code cleanup (16 lines)

- Remove use_joinir_for_array_filter() from env.rs (9 lines)
  Route B is now default, env flag no longer needed

- Remove next_var_id() from ast_lowerer.rs (7 lines)
  Duplicate of alloc_var() method

Cumulative deletion: 51 lines (35 + 16)
Tests: 5/5 Phase 40 PASS

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

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

src/config/env.rs

@@ -265,17 +265,8 @@ pub fn joinir_debug_level() -> u8 {
     0
 }
 
-/// Phase 40-3: array_ext.filter JoinIR経路切り替えフラグ
-///
-/// - OFF (default): 旧AST→MIR + collect_assigned_vars経路
-/// - ON: JoinIR Frontend + JoinFuncMeta + convert_with_meta経路
-///
-/// Set HAKO_JOINIR_ARRAY_FILTER=1 to enable JoinIR route for array_ext.filter.
-pub fn use_joinir_for_array_filter() -> bool {
-    env_bool("HAKO_JOINIR_ARRAY_FILTER")
-}
-
 // VM legacy by-name call fallback was removed (Phase 2 complete).
+// Phase 40-4.1: use_joinir_for_array_filter() removed (Route B now default).
 
 // ---- Phase 11.8 MIR cleanup toggles ----
 /// Core-13 minimal MIR mode toggle. Default ON unless NYASH_MIR_CORE13=0.

src/mir/join_ir/frontend/ast_lowerer.rs

@@ -1038,13 +1038,6 @@ impl AstToJoinIrLowerer {
         id
     }
 
-    /// 次の変数 ID を生成
-    fn next_var_id(&mut self) -> VarId {
-        let id = crate::mir::ValueId(self.next_var_id);
-        self.next_var_id += 1;
-        id
-    }
-
     /// Phase 34-5: expr から「値を計算する JoinIR」と「結果を入れる ValueId」を返す
     ///
     /// ## 設計方針