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/answertime_test.go

@@ -91,6 +91,18 @@ func TestSixteenThousandCookiesAreRefusedWellWithinTheTimeLimit(t *testing.T) {
 	}
 }
 
+func TestHeaderLimitRefusalNamesItsHeuristicCountsAndLimits(t *testing.T) {
+	t.Parallel()
+
+	s := startDiscriminator(t, "", nil)
+	req := s.question(siteA, client, "/")
+	fillHeaders(req, headerFieldLimit+1)
+	do(t, req)
+
+	wantHeuristic(t, s.out.requestLine(t), "header_limits",
+		`{"fields": 101, "fields_limit": 100, "cookies": 0, "cookies_limit": 50}`)
+}
+
 func TestEachHeaderLimitRefusesARequestOverIt(t *testing.T) {
 	t.Parallel()