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. By hand afterwards: the header's spelling, which
the linter requires, and the padding after `waf` in `sww`'s usage
text.

Model: opus-5-5

internal/waf/countries.go

@@ -0,0 +1,35 @@
+package waf
+
+// The heuristic countries. It reads SWW_EXCLUSIVELY_ALLOWED_COUNTRIES.
+
+import (
+	"context"
+	"slices"
+	"time"
+
+	"sneak.berlin/go/smallwebwaf/internal/requestlog"
+)
+
+// countries refuses a request when SWW_EXCLUSIVELY_ALLOWED_COUNTRIES is set and
+// does not hold the client's country as it was looked up. A client without a
+// country, or whose country cannot be found, is refused. Its figures are the
+// country and the setting that refused it.
+type countries struct{}
+
+func (countries) name() string { return "countries" }
+
+func (countries) check(_ context.Context, rq *request, _ time.Time) *verdict {
+	allowed := rq.h.config.ExclusivelyAllowedCountries
+	country := rq.line.Country
+
+	if len(allowed) == 0 || slices.Contains(allowed, country) {
+		return nil
+	}
+
+	return &verdict{
+		action: requestlog.ActionCountryDenied,
+		figures: map[string]any{
+			"country": country, "setting": "SWW_EXCLUSIVELY_ALLOWED_COUNTRIES",
+		},
+	}
+}