refactor(mir): Separate KeepAlive/ReleaseStrong instructions (Phase 287)

Phase 287: KeepAlive/ReleaseStrong 命令分離

## 変更内容（2つの側面）

### 1. 命令セマンティクスの分離
- KeepAlive { values, drop_after: bool } を2命令に分離
  - KeepAlive { values }: スコープ終了での生存維持（PURE）
  - ReleaseStrong { values }: 変数上書き時の強参照解放（WRITE）
- 効果分析の明確化: PURE vs WRITE の境界確定

### 2. VM実行サポート
- handlers/mod.rs: KeepAlive → 完全 no-op
- handlers/mod.rs: ReleaseStrong → release_strong_refs() 呼び出し
- handlers/lifecycle.rs: handle_keepalive() 削除

## 影響範囲
- 10 ファイル修正（31箇所の出現を全変換）
- instruction.rs, builder.rs, lexical_scope.rs, methods.rs,
  display.rs, printer_helpers.rs, query.rs, joinir_id_remapper.rs,
  handlers/mod.rs, handlers/lifecycle.rs

## 検証
- 154/154 quick smoke PASS
- weak テスト回帰なし（weak_upgrade_fail, weak_basic）
- rg -n drop_after src → 0 件（完全除去確認）
- MIRダンプで release_strong/keepalive 正常表示

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

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

src/backend/mir_interpreter/handlers/lifecycle.rs

@@ -3,18 +3,9 @@ use std::collections::HashSet;
 use std::sync::Arc;
 
 impl MirInterpreter {
-    pub(super) fn handle_keepalive(
-        &mut self,
-        values: &[ValueId],
-        drop_after: bool,
-    ) -> Result<(), VMError> {
-        if drop_after {
-            self.release_strong_refs(values);
-        }
-        Ok(())
-    }
-
-    fn release_strong_refs(&mut self, values: &[ValueId]) {
+    /// Phase 287: Release strong references for all values (including SSA aliases)
+    /// This is called by ReleaseStrong instruction for variable overwrite semantics.
+    pub(super) fn release_strong_refs(&mut self, values: &[ValueId]) {
         let mut arc_ptrs: HashSet<*const dyn NyashBox> = HashSet::new();
 
         for value_id in values {

src/backend/mir_interpreter/handlers/mod.rs

@@ -135,8 +135,12 @@ impl MirInterpreter {
             | MirInstruction::BarrierWrite { .. }
             | MirInstruction::Barrier { .. }
             | MirInstruction::Safepoint => {}
-            MirInstruction::KeepAlive { values, drop_after } => {
-                self.handle_keepalive(values, *drop_after)?;
+            // Phase 287: Lifecycle management
+            MirInstruction::KeepAlive { .. } => {
+                // No-op: KeepAlive only affects DCE/liveness analysis
+            }
+            MirInstruction::ReleaseStrong { values } => {
+                self.release_strong_refs(values);
             }
             MirInstruction::Nop => {}
             other => {