fix(edge): suppress dead HTTP/3 advert with Alt-Svc: clear

Caddy enables HTTP/3 by default on any TLS listener and emits
Alt-Svc: h3=":443"; ma=2592000, but UDP/443 is never reachable: the prod
compose maps only "443:443" (TCP) and ufw opens 443/tcp (test contour: the
host caddy publishes only :443/tcp). A client that cached the 30-day advert
tries QUIC first on later opens, gets no response, and waits for the QUIC
attempt to time out before falling back to h2 -- which surfaced as the
Telegram Mini App intermittently hanging on load (a barely-noticeable pause
up to a blank window). The h2/TCP serving path itself is healthy (~10ms TTFB).

Emit Alt-Svc: clear site-wide at the contour caddy so clients actively drop
any cached alternative and stay on h2/h1. This caddy terminates TLS in prod
(the fix target); in the test contour it serves plain :80 and the host caddy
re-stamps its own Alt-Svc, so the live test fix lives in the host caddy. Add
docs/EDGE_HTTP3.md (symptom, diagnosis method, verify, and option B -- serving
h3 for real -- if it recurs) and link it from ARCHITECTURE.md.

docs/ARCHITECTURE.md

@@ -1098,7 +1098,10 @@ Two contours, two secret/variable prefixes (`TEST_` / `PROD_`):
   the **main host** runs the full stack (`docker-compose.yml` + `docker-compose.prod.yml`),
   the **bot host** runs only the bot (`docker-compose.bot.yml`, no VPN — native Bot API
   egress, telemetry off). There is no host caddy, so the contour caddy terminates TLS —
-  `CADDY_SITE_ADDRESS` is the domain and caddy does its own ACME. The gateway **publishes**
+  `CADDY_SITE_ADDRESS` is the domain and caddy does its own ACME. Caddy advertises HTTP/3 by default, but UDP/443 is not exposed (the
+  compose maps only TCP and ufw opens 443/tcp), so the edge emits `Alt-Svc: clear` to keep
+  clients on h2/h1 rather than stall on a dead QUIC path — see [`EDGE_HTTP3.md`](EDGE_HTTP3.md).
+  The gateway **publishes**
   the bot-link `:9443`; the remote bot dials it over mTLS (certs from `PROD_BOTLINK_*`,
   ServerName `gateway`, so TLS validation is independent of the public dial address), holds
   no inbound port, and login is unaffected if that host or the link is down.