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>

apps/tests/assignment_expr_shortcircuit.hako

@@ -0,0 +1,12 @@
+// Phase 152-A: Grouped assignment expression with shortcircuit
+// Test: (x = expr) can be used in conditional expressions
+
+static box Main {
+    main() {
+        local x = 0
+        if ((x = 1) > 0) and true {
+            return x  // Expected: RC 1
+        }
+        return -1
+    }
+}

apps/tests/assignment_expr_simple.hako

@@ -0,0 +1,11 @@
+// Phase 152-A: Grouped assignment expression simple test
+// Test: (x = expr) returns value and assigns to x
+
+static box Main {
+    main() {
+        local x = 0
+        local y = (x = x + 1)
+        // x should be 1, y should be 1
+        return y  // Expected: RC 1
+    }
+}

apps/tests/shortcircuit_and_phi_skip.hako

@@ -1,7 +1,8 @@
 static box Main {
   main(args) {
     local x = 0
-    ((x = x + 1) < 0) && ((x = x + 1) < 0)
+    // Phase 152-A: Use grouped assignment in expression context
+    local result = ((x = x + 1) < 0) && ((x = x + 1) < 0)
     // LHS が false → RHS は評価されず x は 1 のまま
     return x
   }