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_test.go

@@ -90,6 +90,23 @@ func TestCountryLists(t *testing.T) {
 	}
 }
 
+func TestCountryRefusalNamesItsHeuristicCountryAndSetting(t *testing.T) {
+	t.Parallel()
+
+	geojsURL, _ := startGeoJS(t)
+	s, _, _ := startWithClock(t, geojsURL, map[string]string{
+		lookupTimeout: "1h", deniedCountries: "kp", allowedCountries: "de",
+	})
+
+	line := s.get(fromKP, http.StatusForbidden, requestlog.ActionCountryDenied)
+	wantHeuristic(t, line, "countries",
+		`{"country": "KP", "setting": "SWW_DENIED_COUNTRIES"}`)
+
+	line = s.get(unplaced, http.StatusForbidden, requestlog.ActionCountryDenied)
+	wantHeuristic(t, line, "countries",
+		`{"country": "", "setting": "SWW_EXCLUSIVELY_ALLOWED_COUNTRIES"}`)
+}
+
 func TestCountryRefusalKeepsARequestFromTheApp(t *testing.T) {
 	t.Parallel()