feat(joinir): Phase 256 P1.5 - JoinInst::Select 命令サポート（根治）

## 変更内容

Task 1（既完了）: boundary パラメータを bridge 経路全体に伝播
- conversion_pipeline.rs, bridge.rs（複数箇所）, meta.rs, routing_legacy_binding.rs, execute_box.rs

Task 2-A～2-D: Select 命令の実装
- 2-A: MirInstruction::Select バリアント追加
- 2-B: JoinIR Select → MIR Select 直接変換（branch+phi展開廃止）
- 2-C: ⭐ joinir_id_remapper に Select remap case（ValueId変換の根治）
- 2-D: value_collector に Select case

## 根本原因解決

Pattern7 の JoinInst::Select が JoinIR→MIR で未対応
→ ValueId(1002) → ValueId(57) のリマップが行われず
→ "use of undefined value ValueId(57)" エラー

## 現在地

✅ cargo check: 0 errors
⏳ Pattern7/6 VM test（Task 2-E LLVM実装後）

🧠 Generated with Claude Code
Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

src/mir/builder/control_flow/joinir/patterns/conversion_pipeline.rs

@@ -94,9 +94,9 @@ impl JoinIRConversionPipeline {
         );
 
         // Step 2: JoinModule → MirModule conversion
-        // Pass empty meta map since minimal lowerers don't use metadata
+        // Phase 256 P1.5: Pass boundary to bridge for ValueId remapping
         let empty_meta: JoinFuncMetaMap = BTreeMap::new();
-        let mir_module = bridge_joinir_to_mir_with_meta(&join_module, &empty_meta)
+        let mir_module = bridge_joinir_to_mir_with_meta(&join_module, &empty_meta, boundary)
             .map_err(|e| format!("[{}/pipeline] MIR conversion failed: {:?}", pattern_name, e))?;
 
         // Step 3: Log MIR stats (functions and blocks)