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/tagdeny.go

@@ -0,0 +1,30 @@
+package waf
+
+// The heuristic tag_deny. It reads SWW_TAG_DENY.
+
+import (
+	"context"
+	"slices"
+	"time"
+
+	"sneak.berlin/go/smallwebwaf/internal/requestlog"
+)
+
+// tagDeny refuses a request that carries a tag in SWW_TAG_DENY. Its figure is
+// the first such tag, in sorted order.
+type tagDeny struct{}
+
+func (tagDeny) name() string { return "tag_deny" }
+
+func (tagDeny) check(_ context.Context, rq *request, _ time.Time) *verdict {
+	for _, tag := range rq.line.Tags {
+		if slices.Contains(rq.h.config.TagDeny, tag) {
+			return &verdict{
+				action:  requestlog.ActionClientDenied,
+				figures: map[string]any{"tag": tag},
+			}
+		}
+	}
+
+	return nil
+}