fix(mir): SSA違反修正 & StringBox is_space/starts_with実装

Task A: ローカル変数SSA違反修正
- src/mir/builder/stmts.rs: Copy命令で一意ValueId割り当て
- 元のエラー "Invalid value: use of undefined value" 解決
- using_resolver_box.hako が正常動作確認

Task B: StringBox新メソッド実装
- plugins/nyash-string-plugin: is_space/starts_with追加
- M_IS_SPACE (7), M_STARTS_WITH (8) 実装
- string_helpers.hako仕様に準拠

残存問題: do_break()のunreachableブロック生成
→ 次のコミットで修正予定

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

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

src/mir/builder/stmts.rs

@@ -193,18 +193,32 @@ impl super::MirBuilder {
         let mut last_value = None;
         for (i, var_name) in variables.iter().enumerate() {
             let var_id = if i < initial_values.len() && initial_values[i].is_some() {
-                // Use initializer's ValueId directly to avoid SSA aliasing/undefined use
+                // Evaluate the initializer expression
                 let init_expr = initial_values[i].as_ref().unwrap();
                 let init_val = self.build_expression(*init_expr.clone())?;
-                init_val
+
+                // FIX: Allocate a new ValueId for this local variable
+                // and emit a Copy instruction to establish SSA form
+                let var_id = self.value_gen.next();
+
+                self.emit_instruction(crate::mir::MirInstruction::Copy {
+                    dst: var_id,
+                    src: init_val
+                })?;
+
+                // Propagate metadata (type/origin) from initializer to variable
+                crate::mir::builder::metadata::propagate::propagate(self, init_val, var_id);
+
+                var_id
             } else {
                 // Create a concrete register for uninitialized locals (Void)
                 crate::mir::builder::emission::constant::emit_void(self)
             };
+
             self.variable_map.insert(var_name.clone(), var_id);
             last_value = Some(var_id);
         }
-        Ok(last_value.unwrap_or_else(|| self.next_value_id()))
+        Ok(last_value.unwrap_or_else(|| self.value_gen.next()))
     }
 
     // Return statement