IPv4 link-local on the TAP adapter means Windows self-assigned APIPA because
LAN DHCP did not complete through the tunnel. The previous user diagnostic
only said "TAP IP detected", which made a failed DHCP path look neutral
during MVP testing.
Return a full UserDiagnostic from TAP IP classification so IPv4 link-local
can be a warning while normal IPv4 still reports the received DHCP address.
Keep IPv6 link-local neutral because it is expected on many Windows
interfaces and is not evidence of LAN DHCP success or failure.
TESTING.md now tells the operator to troubleshoot 169.254.x.x like
`Waiting for TAP IP`.
Test Plan:
- cargo test -p lanparty-obs
- cargo test -p lanparty-client-win
- cargo test --workspace
- cargo clippy --workspace --all-targets -- -D warnings
- cargo fmt --check
- git diff --check
Refs: PLAN.md diagnostics; TESTING.md MVP guide
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