Rename discriminator to waf everywhere (closes #312)

The command is now `sww waf`, and the package is `internal/waf`. Types,
functions, files, tests, comments, docs, the Dockerfile, the runit
script, `script/`, compose examples, the log field `waf_line` and the
`X-From-Waf` test header follow. No behaviour changes, and no setting
changes name: rproxy already finds the waf through `SWRP_WAF_BACKEND`.

Done with one `sed -i` (Discriminator and DISCRIMINATOR to WAF,
discriminator to waf) over the files `git grep -i discriminator`
listed, as sneak allowed for this rename, then `git mv` for the
package and files.

Model: opus-5-5

internal/waf/health_test.go

@@ -0,0 +1,41 @@
+package waf_test
+
+import (
+	"net/http"
+	"testing"
+
+	"sneak.berlin/go/smallwebwaf/internal/requestlog"
+	"sneak.berlin/go/smallwebwaf/internal/waf"
+)
+
+func TestHealthEndpointCountsTowardNoLimit(t *testing.T) {
+	t.Parallel()
+
+	// With a limit of one request a minute, any request counted before
+	// the last one would have it refused.
+	s, _, _ := startWithClock(t, "", map[string]string{rateLimitPerMinute: "1"})
+
+	const (
+		healthChecks = 3
+		contentType  = "text/plain; charset=utf-8"
+	)
+
+	for range healthChecks {
+		got := s.ownEndpointRequest("", http.MethodGet, waf.HealthPath, "",
+			http.StatusOK)
+
+		if string(got.body) != "ok\n" || got.header.Get("Content-Type") != contentType {
+			t.Errorf("health endpoint answered %q with Content-Type %q, want ok "+
+				"with %q", got.body, got.header.Get("Content-Type"), contentType)
+		}
+	}
+
+	s.get(client, http.StatusOK, requestlog.ActionForward)
+
+	for _, line := range s.out.endedLines(t, healthChecks)[:healthChecks] {
+		if line.ResponseContentType != contentType {
+			t.Errorf("health check's log line has response_content_type %q, "+
+				"want %q", line.ResponseContentType, contentType)
+		}
+	}
+}