refactor(gw-listener): simplify drift detector helpers and listener dispatch

Inline trivial free functions (has_multiple_variants, seen_sender_strings,
missing_sender_strings, address_strings, digest_pair_from_db_digests) at
call sites and delete them. Remove redundant .or_else() in sender_seed_block
computation. Convert if-else address dispatch to match. Add submit() test
helper to compress repetitive observe_submission boilerplate.

Author: Eikix