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/parser/expr/primary.rs

@@ -265,6 +265,12 @@ impl NyashParser {
                 }
             }
             TokenType::LPAREN => {
+                // Phase 152-A: Try grouped assignment first (Stage-3 only)
+                if let Some(assignment) = self.try_parse_grouped_assignment()? {
+                    return Ok(assignment);
+                }
+
+                // Fallback: normal grouped expression
                 self.advance();
                 let expr = self.parse_expression()?;
                 self.consume(TokenType::RPAREN)?;