feat(cli): let --android-app-path supply the bundle id

--bundle-id stays required everywhere else, and an explicit one still
wins, so the apk can never quietly override what was asked for.

Claude-Session: https://claude.ai/code/session_012PVErdr3ZzyUASeVQDWsUc

cmd/sanderling/main.go

@@ -12,6 +12,7 @@ import (
 	"syscall"
 	"time"
 
+	"github.com/priyanshujain/sanderling/internal/android"
 	"github.com/priyanshujain/sanderling/internal/testrun"
 )
 
@@ -60,7 +61,7 @@ func parseTestArgs(args []string, stderr io.Writer) (testOptions, error) {
 	flagSet.SetOutput(stderr)
 	var options testOptions
 	flagSet.StringVar(&options.spec, "spec", "", "path to the TypeScript spec (required)")
-	flagSet.StringVar(&options.bundleID, "bundle-id", "", "target app bundle ID (required)")
+	flagSet.StringVar(&options.bundleID, "bundle-id", "", "target app bundle ID (required, except on android with --android-app-path: the APK names the package itself)")
 	flagSet.StringVar(&options.platform, "platform", "android", "target platform: android, ios, web")
 	flagSet.StringVar(&options.avd, "avd", "", "Android AVD name to boot if no device is connected")
 	flagSet.StringVar(&options.device, "device", "", "Android device serial (from `adb devices`) to target when several are connected")
@@ -85,7 +86,14 @@ func parseTestArgs(args []string, stderr io.Writer) (testOptions, error) {
 		return testOptions{}, errors.New("--spec is required")
 	}
 	if options.bundleID == "" {
-		return testOptions{}, errors.New("--bundle-id is required")
+		if options.platform != "android" || options.androidAppPath == "" {
+			return testOptions{}, errors.New("--bundle-id is required")
+		}
+		packageName, err := android.PackageName(options.androidAppPath)
+		if err != nil {
+			return testOptions{}, err
+		}
+		options.bundleID = packageName
 	}
 	switch options.platform {
 	case "android", "ios", "web":

cmd/sanderling/main_test.go

@@ -88,6 +88,52 @@ func TestParseTestArgs_RequiresBundleID(t *testing.T) {
 	}
 }
 
+// folioAPK is the android package's fixture, borrowed rather than copied so
+// there is one compiled manifest in the tree to keep current.
+const folioAPK = "../../internal/android/testdata/folio.apk"
+
+func TestParseTestArgs_ReadsBundleIDFromTheAPK(t *testing.T) {
+	options, err := parseTestArgs([]string{"--spec", "s.ts", "--android-app-path", folioAPK}, io.Discard)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if options.bundleID != "app.folio" {
+		t.Fatalf("bundle id: got %q, want app.folio", options.bundleID)
+	}
+}
+
+func TestParseTestArgs_ExplicitBundleIDOutranksTheAPK(t *testing.T) {
+	options, err := parseTestArgs([]string{
+		"--spec", "s.ts",
+		"--bundle-id", "app.folio.debug",
+		"--android-app-path", folioAPK,
+	}, io.Discard)
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if options.bundleID != "app.folio.debug" {
+		t.Fatalf("bundle id: got %q, want app.folio.debug", options.bundleID)
+	}
+}
+
+func TestParseTestArgs_APKDoesNotNameTheBundleOffAndroid(t *testing.T) {
+	_, err := parseTestArgs([]string{
+		"--spec", "s.ts",
+		"--platform", "ios",
+		"--android-app-path", folioAPK,
+	}, io.Discard)
+	if err == nil || !strings.Contains(err.Error(), "--bundle-id") {
+		t.Fatalf("expected missing --bundle-id error, got %v", err)
+	}
+}
+
+func TestParseTestArgs_UnreadableAPKIsReported(t *testing.T) {
+	_, err := parseTestArgs([]string{"--spec", "s.ts", "--android-app-path", "testdata/absent.apk"}, io.Discard)
+	if err == nil || !strings.Contains(err.Error(), "testdata/absent.apk") {
+		t.Fatalf("expected an error naming the apk, got %v", err)
+	}
+}
+
 func TestParseTestArgs_AVDIsOptional(t *testing.T) {
 	options, err := parseTestArgs([]string{"--spec", "s.ts", "--bundle-id", "com.example"}, io.Discard)
 	if err != nil {