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/mir/builder/exprs.rs

@@ -134,6 +134,13 @@ impl super::MirBuilder {
                 }
             }
 
+            // Phase 152-A: Grouped assignment expression (x = expr)
+            // Stage-3 only. Value/type same as rhs, side effect assigns to lhs.
+            // Reuses existing build_assignment logic, returns the SSA ValueId.
+            ASTNode::GroupedAssignmentExpr { lhs, rhs, .. } => {
+                self.build_assignment(lhs.clone(), *rhs.clone())
+            }
+
             ASTNode::Index { target, index, .. } => {
                 self.build_index_expression(*target.clone(), *index.clone())
             }

src/mir/builder/vars.rs

@@ -24,6 +24,10 @@ pub(super) fn collect_free_vars(
             collect_free_vars(target, used, locals);
             collect_free_vars(value, used, locals);
         }
+        // Phase 152-A: Grouped assignment expression
+        ASTNode::GroupedAssignmentExpr { rhs, .. } => {
+            collect_free_vars(rhs, used, locals);
+        }
         ASTNode::BinaryOp { left, right, .. } => {
             collect_free_vars(left, used, locals);
             collect_free_vars(right, used, locals);