feat(privacy): emit ExternalInvoke event on external invoke (#855)

Emit an ExternalInvoke event from _apply_invoke_and_deposits, keyed on
contract_address + selector, covering both privacy_invoke and
privacy_invoke_with_computation. Calldata is intentionally not emitted; the
selector distinguishes the two paths, and both target and selector are already
visible in the public call trace, so this leaks nothing new.

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