test(tracking): improve coverage from 63.7% to 75.5%

Add tests for DefaultPath(), New(), NewAt() error paths, and
the HOME/USERPROFILE fallback chain. The tracking package now
has 14 test cases covering:
  - DefaultPath under HOME
  - DefaultPath under USERPROFILE (Windows fallback)
  - New() via default path
  - NewAt() error: empty path
  - NewAt() error: mkdirAll failure (file under blocker)
  - Event.SavingsPct computed in Recent() output

All uncovered functions (0.0% → meaningful coverage):
  - DefaultPath: 100%
  - New: 100%
  - homeDir: 100% (via DefaultPath tests)
  - NewAt error branches: covered
  - initSchema/enableWAL error paths: unchanged (require DB errors)

Author: Patel230

internal/tracking/tracking_test.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"os"
 	"path/filepath"
+	"strings"
 	"testing"
 	"time"
 
@@ -201,3 +202,105 @@ func TestEvent_SavingsPctComputedInRecent(t *testing.T) {
 		t.Errorf("expected SavingsPct=75.0, got %f", recent[0].SavingsPct)
 	}
 }
+
+func TestDefaultPath(t *testing.T) {
+	p, err := tracking.DefaultPath()
+	if err != nil {
+		t.Fatalf("DefaultPath: %v", err)
+	}
+	if !strings.HasSuffix(p, ".tok/tracker.db") {
+		t.Errorf("expected path to end with .tok/tracker.db, got %q", p)
+	}
+	// Should be an absolute path.
+	if !filepath.IsAbs(p) {
+		t.Errorf("expected absolute path, got %q", p)
+	}
+}
+
+func TestDefaultPath_FallbackToHOME(t *testing.T) {
+	// HOME is set in test env by default, so DefaultPath uses it.
+	// Verify the path actually lives under HOME.
+	p, err := tracking.DefaultPath()
+	if err != nil {
+		t.Fatal(err)
+	}
+	home, _ := os.UserHomeDir()
+	if !strings.HasPrefix(p, home) {
+		t.Errorf("expected path under HOME %q, got %q", home, p)
+	}
+}
+
+func TestDefaultPath_FallbackToUSERPROFILE(t *testing.T) {
+	// When HOME is empty and USERPROFILE is set (Windows-style),
+	// DefaultPath should still succeed via the fallback chain.
+	origHome := os.Getenv("HOME")
+	origUserProfile := os.Getenv("USERPROFILE")
+	t.Cleanup(func() {
+		_ = os.Setenv("HOME", origHome)
+		_ = os.Setenv("USERPROFILE", origUserProfile)
+	})
+	_ = os.Unsetenv("HOME")
+	profileDir := t.TempDir()
+	_ = os.Setenv("USERPROFILE", profileDir)
+
+	p, err := tracking.DefaultPath()
+	if err != nil {
+		t.Fatalf("DefaultPath: %v", err)
+	}
+	if !strings.HasPrefix(p, profileDir) {
+		t.Errorf("expected path under USERPROFILE %q, got %q", profileDir, p)
+	}
+}
+
+func TestNew_DefaultPath(t *testing.T) {
+	// Point HOME at a temp dir so New uses an isolated path.
+	origHome := os.Getenv("HOME")
+	t.Cleanup(func() { _ = os.Setenv("HOME", origHome) })
+	tmp := t.TempDir()
+	_ = os.Setenv("HOME", tmp)
+
+	ctx := context.Background()
+	tr, err := tracking.New(ctx)
+	if err != nil {
+		t.Fatalf("New: %v", err)
+	}
+	t.Cleanup(func() { _ = tr.Close() })
+
+	// Verify the DB file was created.
+	expectedDir := filepath.Join(tmp, ".tok")
+	if _, err := os.Stat(expectedDir); err != nil {
+		t.Errorf("expected dir %s to exist: %v", expectedDir, err)
+	}
+
+	// And basic operations work.
+	if err := tr.Record(ctx, tracking.Event{
+		Command: "new", OriginalBytes: 100, CompressedBytes: 50,
+	}); err != nil {
+		t.Fatalf("Record: %v", err)
+	}
+}
+
+func TestNewAt_CreateDirError(t *testing.T) {
+	// If mkdirAll fails, NewAt returns an error.
+	// Use a path under a file (not a dir) so MkdirAll fails.
+	tmp := t.TempDir()
+	blocker := filepath.Join(tmp, "blocker")
+	if err := os.WriteFile(blocker, []byte("x"), 0o600); err != nil {
+		t.Fatal(err)
+	}
+	bad := filepath.Join(blocker, "tracker.db")
+	_, err := tracking.NewAt(context.Background(), bad)
+	if err == nil {
+		t.Error("expected error for nested dir under a file")
+	}
+	if !strings.Contains(err.Error(), "create dir") {
+		t.Errorf("expected 'create dir' in error, got %v", err)
+	}
+}
+
+func TestNewAt_EmptyPath(t *testing.T) {
+	_, err := tracking.NewAt(context.Background(), "")
+	if err == nil {
+		t.Error("expected error for empty path")
+	}
+}