feat(parser): Phase 152-A - Grouped assignment expression (箱化モジュール化)

Implement Stage-3 grouped assignment expression `(x = expr)` following
the 箱化モジュール化 (modular box) pattern established in Phase 133/134.

**Implementation**:
- AssignmentExprParser module (Rust: 183 lines)
  - src/parser/stage3/assignment_expr_parser.rs (+183 lines)
  - src/parser/stage3/mod.rs (+9 lines)
- AST node addition: GroupedAssignmentExpr
  - src/ast.rs (+7 lines)
  - src/ast/utils.rs (+9 lines)
- MIR lowering via 1-line delegation
  - src/mir/builder/exprs.rs (+5 lines)
  - src/mir/builder/vars.rs (+4 lines)
- Parser integration via 1-line delegation
  - src/parser/expr/primary.rs (+6 lines)
  - src/parser/mod.rs (+1 line)

**Test Results**: 3/3 PASS
- assignment_expr_simple.hako: RC 1 ✅
- assignment_expr_shortcircuit.hako: RC 1 ✅
- shortcircuit_and_phi_skip.hako: RC 1 ✅ (updated to use expression context)

**Stage-3 Gate**: No impact on Stage-2/legacy
- NYASH_FEATURES=stage3 required
- Pattern: '(' IDENT '=' expr ')'
- Value/type same as rhs, side effect assigns to lhs

**箱化モジュール化パターン**:
- Dedicated module for assignment expression parsing
- Clear responsibility separation
- 1-line delegation for integration
- Testability improvement
- Follows Phase 133/134-A/134-B pattern

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

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

src/ast.rs

@@ -523,6 +523,15 @@ pub enum ASTNode {
         span: Span,
     },
 
+    /// Stage-3: 括弧付き代入式: (x = expr) - Phase 152-A
+    /// 値・型は右辺と同じ、副作用として左辺に代入
+    /// 使用例: local y = (x = x + 1), if (x = next()) != null { }
+    GroupedAssignmentExpr {
+        lhs: String,           // 変数名
+        rhs: Box<ASTNode>,     // 右辺式
+        span: Span,
+    },
+
     /// メソッド呼び出し: object.method(arguments)
     MethodCall {
         object: Box<ASTNode>,