router: a Russian verb is a whole sentence, not thin evidence

The clarify gate thinned any one-word utterance to 0.3 confidence, which
trips the stage-3 gate and comes back as "не совсем поняла". That is an
English intuition. Russian packs subject, tense and gender into one word,
so "поужинал" is a complete report and "привет" a complete greeting, and
both got clarified.

thinSingleToken keeps the rule for bare nominals, where it is real ("вода"
is a fact-or-query coin flip), and spares two classes: a closed lexicon of
social and control singles, and any token carrying a verb ending. Both
tests are offline.

Fixture: false clarifies 3 → 2, intent-only 74.0% → 75.3%, full accuracy
unchanged at 70.1%, missed clarify still 1.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TrVSBKe3RFDF4fGYKWYQnX

internal/router/singletoken_test.go

@@ -0,0 +1,33 @@
+package router
+
+import "testing"
+
+func TestThinSingleTokenThinsBareNominals(t *testing.T) {
+	// The case the rule exists for: one noun, no way to tell what was asked.
+	for _, w := range []string{"вода", "бэкап", "нексус", "почта", "backup"} {
+		if !thinSingleToken(w) {
+			t.Errorf("thinSingleToken(%q) = false, want true", w)
+		}
+	}
+}
+
+func TestThinSingleTokenSparesCompleteUtterances(t *testing.T) {
+	// Regression: every one of these used to be answered with
+	// "не совсем поняла — можешь переформулировать?".
+	for _, w := range []string{
+		"привет", "Привет!", "спасибо", "да", "нет", "стоп", "hello", "yes",
+		"поужинал", "проснулась", "устал", "выспался", "договорились",
+	} {
+		if thinSingleToken(w) {
+			t.Errorf("thinSingleToken(%q) = true, want false", w)
+		}
+	}
+}
+
+func TestThinSingleTokenIgnoresMultiWord(t *testing.T) {
+	for _, s := range []string{"выпил воды", "что там с бэкапом", ""} {
+		if thinSingleToken(s) {
+			t.Errorf("thinSingleToken(%q) = true, want false", s)
+		}
+	}
+}