feat(obs): warn on missing client readiness

The friendly client diagnostics reported relay/gateway/IP/broadcast signals,
but they still hid two important readiness failures in the dense status line:
the relay route was not pinned, or no TAP adapter was available. Those are the
kind of states a CLI user or future GUI needs surfaced immediately.

Add user diagnostics for an unpinned relay route, a missing TAP adapter, and a
TAP adapter that is present but still has no IP address. The existing healthy
path stays concise: once the TAP has an address, the diagnostics continue to
report that address instead of also saying that the adapter is ready.

README now mentions that the short diagnostics include route and TAP readiness
warnings.

Test Plan:
- cargo fmt --check
- cargo test -p lanparty-obs -p lanparty-client-win
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
- git diff --check
- git diff --cached --check

Refs: PLAN.md Logging / diagnostics

README.md

@@ -219,8 +219,9 @@ LAN-gateway presence, route-pinning, QUIC datagram budget, TAP status/IP,
 broadcast frame flow, frame/datagram counters, and drops. The periodic
 diagnostics refresh the TAP unicast IP so DHCP results that arrive after
 bridging starts become visible in later status lines. Each snapshot also emits
-short user-facing lines such as relay/gateway connection status, DHCP address
-presence, and broadcast-flow confirmation when those signals are observed.
+short user-facing lines such as relay/gateway connection status, relay-route
+and TAP readiness warnings, DHCP address presence, and broadcast-flow
+confirmation when those signals are observed.
 Relay lifecycle events are logged as they arrive, including gateway joins and
 peer leaves. The client remembers peer identities from join and catch-up events
 so later leave logs can identify a disconnected LAN gateway or client MAC when