feat(joinir): Phase P2-P4 Loop Pattern boxification

- P2: Create loop_patterns/ module structure
  - common.rs: shared helpers (ParsedProgram, LoopContext, etc.)
  - simple.rs: generic loop lowering
  - filter.rs, print_tokens.rs: delegate to simple
  - break_pattern.rs, continue_pattern.rs: new modules

- P3: Create LoopFrontendBinding layer
  - Separate function name detection from pattern lowering
  - detect_loop_pattern() for pattern classification

- P4: Break/Continue pattern migration
  - Move Break/Continue handling from loop_patterns_old.rs
  - Add LoopPattern::Break and LoopPattern::Continue variants

- Design: 3-layer architecture
  - LoopFrontendBinding: function name → LoopPattern
  - loop_patterns: LoopPattern → JoinIR
  - Bridge/VM: Box semantics

All 56 JoinIR tests pass.

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

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

src/mir/join_ir/frontend/ast_lowerer/mod.rs

@@ -30,7 +30,9 @@ mod context;
 mod expr;
 mod if_in_loop;
 mod if_return;
+mod loop_frontend_binding;
 mod loop_patterns;
+mod loop_patterns_old;
 mod nested_if;
 mod read_quoted;
 mod stmt_handlers;
@@ -83,18 +85,20 @@ impl AstToJoinIrLowerer {
             .as_str()
             .expect("Function must have 'name'");
 
-        // 3. 関数名で分岐（Phase 34-2/34-3/34-4/34-5/34-7/34-8/41-4/45）
-        // test/local/_read_value_from_pair: If Return pattern
-        // simple: Loop pattern (Phase 34-7/34-8)
-        // parse_loop: Phase 41-4 NestedIfMerge pattern
-        // read_quoted_from: Phase 45 Guard if + Loop with break + accumulator
+        // 3. 関数名で分岐（Phase P3: LoopFrontendBinding 層導入）
+        //
+        // パターン分類:
+        // - If Return pattern: test/local/_read_value_from_pair
+        // - Loop pattern: simple 等 → LoopFrontendBinding 経由
+        // - NestedIfMerge pattern: parse_loop (dev flag gated)
+        // - ReadQuoted pattern: read_quoted_from (dev flag gated)
         match func_name {
             "test" | "local" | "_read_value_from_pair" => {
                 self.lower_if_return_pattern(program_json)
             }
-            "simple" => {
-                // Phase 34-8: Loop パターンの詳細分析（break/continue 検出）
-                self.lower_loop_with_break_continue(program_json)
+            "simple" | "filter" | "print_tokens" | "map" | "reduce" | "fold" => {
+                // Phase P3: LoopFrontendBinding 層経由でディスパッチ
+                loop_frontend_binding::lower_loop_by_function_name(self, program_json)
             }
             "parse_loop" => {
                 // Phase 41-4: NestedIfMerge pattern (dev flag gated)