Class private: compound assignment and update expressions with private members not implemented

[dowdiness]

Problem

obj.#x += 1 (compound assignment) and obj.#x++ (update expression) on private fields/methods are not implemented. PR #532 added obj.#x = val (plain assignment) via a new PrivateMemberAssign AST variant, but compound and update paths bypass that variant entirely.

What works

class C {
  #x = 1;
  setX(v) { this.#x = v; }     // plain assignment → PrivateMemberAssign → set_private_field ✓
  getX() { return this.#x; }   // PrivateMember read → get_private_field ✓
}

What fails

class C {
  #x = 1;
  add(v) { this.#x += v; }    // CompoundAssign(op, PrivateMember(...), ...) → no handler
  inc() { this.#x++; }         // UpdateExpr(_, PrivateMember(...), ...) → no handler
}

Root cause

The parser produces CompoundAssign(AddAssign, PrivateMember(obj, "x"), value, loc) for obj.#x += val and UpdateExpr(PlusPlus, PrivateMember(obj, "x"), false, loc) for obj.#x++. These are dispatched to eval_compound_assign (in operators.mbt) and eval_update (in eval_expr.mbt) respectively — neither has a PrivateMember arm for the LHS target.

Required changes

1. interpreter/runtime/operators.mbt: eval_compound_assign needs a PrivateMember arm that:

   • Evaluates the LHS PrivateMember(obj, name)
   • Gets [[PrivateBrand]] from env
   • Reads the current value via get_private_field
   • Computes the result with eval_binary_op
   • Writes back via set_private_field

2. interpreter/runtime/eval_expr.mbt: eval_update (or its UpdateExpr dispatch) needs a PrivateMember arm that:

   • Evaluates the LHS
   • Gets brand, reads, updates, writes back

Reference

PR #532 (plain assignment implementation), issue #528 (parent tracking issue).