[nooga]

Part of #560 (class private-name cluster).

Problems

• Private names compiled to "K:name" keys per class declaration, so every evaluation of the same class shared them. Instances made by one evaluation passed another evaluation's brand checks: c1.access(c2) returned instead of throwing a TypeError.
• Field initializers ran before private methods and accessors were installed, so a = this.#m() threw.
• Private fields could not be added to a Proxy returned from a base constructor.

Fix

• Per-evaluation private names. Each class evaluation pushes a PrivateEnv node for K (new OpPushPrivateEnv / OpPopPrivateEnv).
  • Closures created inside the class body capture the node chain (ClosureObject.PrivateEnv).
  • The class-evaluation code itself runs in the enclosing frame and sees the node through a stack tagged with frame depth. That stack is trimmed at frame push, like the with-object stack.
  • Private keys resolve through the innermost node for their K. The first evaluation keeps the compiled key, so a class evaluated once costs nothing extra. Later evaluations get a suffixed key, cached per node (pkg/vm/private_env.go).
• Initialization order. The constructor's injected initializers now add private methods and accessors before any field initializer, as InitializeInstanceElements requires. Accessor emission order is now sorted, so it is deterministic.
• Proxies. ProxyObject gets its own private-element store, which the private ops use without touching the handler.

Testing

• New smoke test tests/scripts/class_private_names_per_evaluation.ts. TestScripts is green.
• test262 (language + built-ins, vs main): +20, no regressions.

Five private-related class tests still fail for separate reasons:

• decorator-member-expr-private-identifier
• privatefieldput-primitive-receiver
• privatefieldset-evaluation-order-1
• privatefieldset-typeerror-10
• super-access-inside-a-private-method

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