Fix shared singleton mutation in OneTime bindable member Get methods

[VPDPersonal]

Summary

• OneTimeBindableMember<T>.Get(), OneTimeStructBindableMember<T>.Get(), and OneTimeEnumBindableMember<T>.Get() all mutated a shared static _instance and returned it. Any caller that retained the reference across two Get() calls on the same type would silently observe the second value instead of the first.
• Replaced the singleton pattern with fresh instance allocation per call. Value is now set via constructor rather than post-construction mutation, making each returned instance independently immutable.
• Allocation cost is negligible: OneTime bindings fire exactly once per bind cycle, so this is a one-shot heap allocation per binding — identical in practice to what was already happening at bind time.

Notes for review

The Value property on OneTimeStructBindableMember<T, TBoxed> was changed from { get; private protected set; } to { get; } (init-only via constructor). This is a tighter contract that prevents any future accidental mutation from derived types.

No generated code changes are needed — the generator emits SomeType.Get(memberName) whose result is consumed immediately, so the fix is transparent.

Linked issues

N/A