Closes the loop on Viktor's ask to find the t3 disconnect root cause and
definitively rule infra in or out. Server logs alone cannot separate
'client network broke' from 'Cloudflare/tunnel broke' from 't3-serve
stalled' — every cause collapses into the same 20s-watchdog reconnect.
The t3-probe (stacks/t3code) holds three permanent legs that differ only
in path segment: 'cloudflare' (WS via DoH-resolved public DNS -> WAN ->
CF edge -> tunnel -> Traefik -> dispatch), 'internal' (same WS pinned to
the Traefik LB, no Cloudflare), 't3serve' (HTTP straight to the serve
process). Whichever leg drops convicts its segment; all legs clean while
a user drops exonerates infra with data. Dispatch gains an
unauthenticated /probe/ws echo + /probe/healthz (gorilla/websocket,
test-first) behind an auth=none path carve-out, guarded by the
authentik-walloff probe.
Also starts scraping devvm's node_exporter (job 'devvm') — it ran
unscraped, so the box whose memory/IO stalls cause the drops had zero
pressure history. Alerts T3ProbeLegDown + T3ProbeDropBurst; runbook
docs/runbooks/t3-drop-attribution.md.
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