review: require .2da suffix on race feats tables, shrink rule test compare

Review of #94 branch: tighten racialUsableFeatRules to match the spec's
'race_feat_*.2da' (prefix alone would admit a non-2DA dataset), and
replace the test's length-check-plus-loop with one reflect.DeepEqual.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

internal/topdata/racial_feat_rules_test.go

@@ -47,12 +47,8 @@ func TestRacialUsableFeatRules(t *testing.T) {
 		{Feat: "feat:tiefling/darkness", List: "3", GrantedOnLevel: "99", OnMenu: "1"},
 	}
 
-	if len(got) != len(want) {
-		t.Fatalf("rule count: got %d, want %d (%+v)", len(got), len(want), got)
-	}
-	for i := range want {
-		if !reflect.DeepEqual(got[i], want[i]) { // slice is sorted by Feat, so order is deterministic
-			t.Fatalf("rule %d: got %+v, want %+v", i, got[i], want[i])
-		}
+	// Slice is sorted by Feat, so order is deterministic.
+	if !reflect.DeepEqual(got, want) {
+		t.Fatalf("rules: got %+v, want %+v", got, want)
 	}
 }