Reverts onDiscoveredContact to the upstream 4-param signature, adding a
separate virtual onDiscoveredAdvert(bool was_flood) with a default no-op
implementation. Only MyMesh overrides the new virtual; simple_secure_chat
and any future implementors need no changes on upstream merges.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>