Add minimal compilation smoke test; tidy go.mod

Add a smoke test that references the package's exported surface so go
test fails if it stops compiling. Run go mod tidy to promote cobra to
a direct dependency.

internal/bsdaily/bsdaily_test.go

@@ -0,0 +1,21 @@
+package bsdaily
+
+import "testing"
+
+// TestCompiles is a minimal smoke test that references the package's exported
+// surface so that `go test` fails if the package stops compiling. It does not
+// touch the filesystem or any of the hard-coded production paths.
+func TestCompiles(t *testing.T) {
+	if DBFilename == "" || WALFilename == "" || SHMFilename == "" {
+		t.Fatal("expected database filename constants to be set")
+	}
+	if SnapshotBase == "" || TmpBase == "" || DailiesBase == "" {
+		t.Fatal("expected base path constants to be set")
+	}
+	if MinTmpFreeBytes == 0 || MinDailiesFreeBytes == 0 {
+		t.Fatal("expected free-space thresholds to be set")
+	}
+	if ErrNoPosts == nil {
+		t.Fatal("expected ErrNoPosts sentinel to be set")
+	}
+}