Mesh probe fixtures: tailnet edge cases #25

What you'll build: additional recorded fixtures for the read-only Tailscale/Headscale probes covering edge cases — empty tailnet, expired node key, unreachable control server.

Where: adapters/mesh/ (follow the existing fixture pattern).

Expected behavior: each fixture drives the probe to a deterministic, fail-closed result; no live API calls in tests.

How to test: go test ./adapters/... passes; new cases visible in test names.

Mentor: @DBarr3