The host-config seam: how Tank reads the host's network facts without owning them — the uplink a macvlan attaches to, and the DNS servers a container inherits.

Tank must share these without dragging in any host-networking dependency, so Tank.Host is a behaviour: an adapter contract. The adapter is chosen by config and reached through this module's facade — callers say Tank.Host.uplink() and never name the adapter:

config :tank, host: Tank.Host.Static   # the default

Adapters:

• Tank.Host.Static — shipped here, the v1 default: uplink + DNS straight from config. Runs anywhere.
• A consumer-provided adapter — lives in the consumer, never in Tank's deps: reads the host's own network manager. (later)
• Tank.Host.Linx — auto-detect uplink + DNS from rtnetlink and /etc/resolv.conf; a zero-config default. (later)

The contract is intentionally just what Tank core consumes today: the uplink (resolving a NIC's parent: :auto) and the DNS list (the pod's /etc/resolv.conf when it declares no dns:). It will grow host IP facts and change-notifications when an adapter that has them needs them.