Adds a dedicated test file that drives the small private mapping helpers
inside `Towerops.Topology` via their public surface:
- `role_to_type_atom/1`: every device_role string (router, core_router,
switch, distribution_switch, access_switch, access_point, cpe,
backhaul, backhaul_radio, server, firewall, ups, other, unknown
fallback) is exercised through `device_to_node` (via
`get_topology_for_map/2`) and again through `get_node_detail/2`.
- `link_type_priority/1`: each priority pair (lldp/cdp/mac_match/
arp_inference) is exercised via `merge_bidirectional_edges/1` by
upserting reciprocal links of differing types and asserting the
higher-priority `link_type` survives the merge.
- `evidence_type_to_link_type/1`: every evidence_type clause
(lldp_neighbor, cdp_neighbor, mac_on_interface, arp_entry,
wireless_registration, plus the catch-all) is hit via
`upsert_link/1` → `process_evidence_for_link/2`.
Coverage: 86.28% → 86.42% (+0.14%).
Graham McIntire
fc20d60838