Rename internal/proxy to internal/servewaf (closes #173)

The package passes nothing on since the one-process mode went: it answers rproxy's questions, takes its reports and answers servewaf's own endpoints. It is now named after the command it serves, with `git mv`, and its constructor is `servewaf.New`. `SPEC.md` and the other documents follow. No change of behaviour.

Model: opus-5-5

README.md

@@ -689,8 +689,8 @@ aside as `<name>.bad`, and never stops the running process (see
   health check.
 - `internal/config`: reads the settings, the one place they are read.
 - `internal/rproxy`: the reverse proxy in front of one app.
-- `internal/proxy`: `servewaf`'s server, which runs the checks and answers its
-  own endpoints.
+- `internal/servewaf`: `servewaf`'s server, which runs the checks and answers
+  its own endpoints.
 - `internal/ratelimit`, `internal/bans`, `internal/rules`, `internal/waf`,
   `internal/lookup`, `internal/reputation`, `internal/anomaly` and
   `internal/alerts`: one check or source each.