Mirrors cache-bust mitigation (2026-09-18)

Issue: /channel-hub/mirrors.html was briefly served in two different versions (stale vs fixed) by CDN edges.
Mitigation: internal hub links now use a versioned query param: /channel-hub/mirrors.html?v=20260918-3

This receipt records:
- A probe summary showing whether multiple bodies are observed at the BASE URL and at versioned URLs.
- A Pages stability gate run showing repeated 200 OK across key hub pages and the v=20260918-3 mirrors URL.

Related commits:
- Cache-bust links v=20260918-2: 3773421
- Bump to v=20260918-3: 9d027dd