docs: add callback retention example to Duplicating stubs

[SAY-5]

Refs #30.

In #30, @kentonv (member) said "I agree we could use more examples of callbacks." The follow-up comment showed the typical pitfall: a setCallback(cbfunc) method that stores cbfunc and invokes it later from a timer throws "Attempted to use RPC stub after it has been disposed" unless the stub is duplicated with .dup().

This adds a short subsection to "Duplicating stubs" in the README with:

• the bidirectional-callback pattern that motivates .dup(),
• a 12-line RpcTarget example that stores cbfunc.dup() and disposes it from [Symbol.dispose],
• a note that the same rule applies in the reverse direction (client retaining a stub returned by the server).

README only. No code changes.

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