feat: brain get/stats/eval call internal/brain, not Python (#18)

Read path is cgo like search. Control questions live in rank so CI can
test them without ladybug. Python bin/kb/{get,stats,eval} stays the
runner fallback.

internal/brain/rank/evalq_test.go

@@ -0,0 +1,17 @@
+package rank
+
+import "testing"
+
+func TestEvalQuestionsAreThreeAndThreshold(t *testing.T) {
+	if EvalRecallThreshold != 0.95 {
+		t.Fatalf("threshold = %v", EvalRecallThreshold)
+	}
+	if len(EvalQuestions) != 3 {
+		t.Fatalf("questions = %d, want 3", len(EvalQuestions))
+	}
+	for _, q := range EvalQuestions {
+		if q.Query == "" || q.Fragment == "" {
+			t.Fatalf("empty control: %+v", q)
+		}
+	}
+}