discriminator: each decision heuristic is its own module (closes #195)

Each discriminator check is now a type in its own file behind one small interface, listed in `heuristics.go`. Adding a heuristic is one file and one line. Behaviour does not change.

Refusal and ban log lines gain `heuristic` and `heuristic_figures`. A ban after the answer, by `refusal_limit` or `byte_limits`, logs `after_answer_heuristic` and its figures, so one line can name both.

Deviation: four lists, not the planned three, so `refusal_limit` still bans at the decision.
Rule suppressed: `//nolint:lll` on one struct tag.

Model: opus-5-5

internal/discriminator/countries.go

@@ -1,21 +1,42 @@
 package discriminator
 
+// The heuristic countries. It reads SWW_DENIED_COUNTRIES and
+// SWW_EXCLUSIVELY_ALLOWED_COUNTRIES.
+
 import (
+	"context"
 	"slices"
+	"time"
+
+	"sneak.berlin/go/smallwebwaf/internal/requestlog"
 )
 
-// countryDenied reports whether the country lists refuse the request, by
-// the client's country as it was looked up. A client without a country,
-// or whose country cannot be found, is refused only by
-// SWW_EXCLUSIVELY_ALLOWED_COUNTRIES.
-func (rq *request) countryDenied() bool {
+// countries refuses a request by the client's country as it was looked up. A
+// client without a country, or whose country cannot be found, is refused only
+// by SWW_EXCLUSIVELY_ALLOWED_COUNTRIES. 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 {
 	denied := rq.h.config.DeniedCountries
 	allowed := rq.h.config.ExclusivelyAllowedCountries
 	country := rq.line.Country
 
-	if slices.Contains(denied, country) {
-		return true
+	var setting string
+
+	switch {
+	case slices.Contains(denied, country):
+		setting = "SWW_DENIED_COUNTRIES"
+	case len(allowed) > 0 && !slices.Contains(allowed, country):
+		setting = "SWW_EXCLUSIVELY_ALLOWED_COUNTRIES"
+	default:
+		return nil
 	}
 
-	return len(allowed) > 0 && !slices.Contains(allowed, country)
+	return &verdict{
+		action:  requestlog.ActionCountryDenied,
+		figures: map[string]any{"country": country, "setting": setting},
+	}
 }