Documentation: protocol: document new static master protocol

Documentation/protocol/http-server-list.md

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+# Static HTTP server list
+
+Replacement for the UDP `S2M_SCAN_REQUEST` / `M2A_SERVERSLIST` exchange
+described in [02-connectionless.md](02-connectionless.md). Read-only HTTP,
+no NAT punching, no filtering or pagination in the request.
+
+## URL
+
+`xashcomm.lst` carries a base URL per source:
+
+```
+masterstatic http://master.example.org/server-list
+```
+
+The engine appends `/v1/servers/<gamedir>` and `GET`s the result. For
+`gamedir = valve`:
+
+```
+GET http://master.example.org/server-list/v1/servers/valve
+```
+
+Trailing slashes on the base URL are stripped. Multiple `masterstatic`
+lines are allowed; results are merged.
+
+The request is bare: no body, no auth, no cookies, no compressed encodings.
+Only the standard `User-Agent` is set. `POST` / `PUT` / `DELETE` are not
+used; servers register out of band.
+
+## Response
+
+UTF-8 text, tokenized with `COM_ParseFileSafe` (whitespace separates,
+`//` and `#` start line comments, `"..."` quotes a token). One directive
+per record:
+
+* `ip <address>` — Xash3D server (protocol 49).
+* `gs <address>` — GoldSrc server (protocol 48).
+
+`<address>` is parsed by `NET_StringToAdr` (`1.2.3.4:27015`,
+`[2001:db8::1]:27015`, hostnames). Port defaults to `27015`. Unknown
+directives are skipped together with one operand so new keywords can be
+added without breaking older clients.
+
+`Content-Type` is not inspected, `text/plain; charset=utf-8` expected.
+
+### Example
+
+```
+# diffusion servers
+ip 192.0.2.10:27015
+ip [2001:db8::1]:27015
+gs 198.51.100.5:27015
+```
+
+A file with zero records is valid and represents an empty list.
+
+### Versioning
+
+The `/v1/` segment is fixed in this revision. A future protocol revision
+adds a sibling `/v2/...` resource without breaking older clients.
+
+## Client behaviour
+
+Each `ip` / `gs` record from the response triggers a probe to the listed
+address, identical to a server discovered through the UDP master. DNS
+errors, non-200 responses, and malformed bodies are reported to the
+console.
+
+## Server behaviour
+
+Any static HTTP server works. A typical setup is a periodic job that
+probes a set of known addresses and writes `v1/servers/<gamedir>`.