fix(runner): surface focus-tap errors in InputText (#12)

* fix(runner): surface focus-tap errors in InputText action

A failed Tap/TapSelector before InputText was swallowed, so text typed
into the wrong field (or no field) still reported success. Return the
error so the step fails explicitly.

* feat(sample-app): add username EditText and snapshot

Gives the spec a real EditText target (content-desc: username_field)
so the InputText action path can be exercised end-to-end. The typed
value is mirrored into MainActivity.username and surfaced as the
"username" snapshot for spec assertions.

* feat(sample-app): exercise InputText action against username field

Adds typeUsername action and usernameNeverShrinks property to the
sample spec, and extends the integration test to assert the bundled
spec emits an InputText(desc:username_field, "alice") action and that
the property correctly violates when a snapshot reports a shorter
string.

internal/runner/runner_test.go

@@ -3,6 +3,7 @@ package runner
 import (
 	"context"
 	"encoding/json"
+	"errors"
 	"net"
 	"os"
 	"path/filepath"
@@ -234,6 +235,35 @@ func TestRunner_RecordsScreenFieldFromSnapshot(t *testing.T) {
 	}
 }
 
+func TestApplyAction_InputTextSurfacesFocusTapError(t *testing.T) {
+	t.Run("selector focus tap fails", func(t *testing.T) {
+		driverMock := mockdriver.New()
+		driverMock.Failures[mockdriver.ActionTapSelector] = errors.New("adb unreachable")
+		action := verifier.Action{Kind: verifier.ActionKindInputText, On: "id:username", Text: "alice"}
+
+		err := applyAction(context.Background(), driverMock, action, nil)
+		if err == nil {
+			t.Fatalf("expected focus tap failure to surface, got nil")
+		}
+		if containsAction(driverMock.Actions(), mockdriver.ActionInputText, "") {
+			t.Errorf("InputText must not run after focus tap failed: %v", driverMock.Actions())
+		}
+	})
+	t.Run("coordinate focus tap fails", func(t *testing.T) {
+		driverMock := mockdriver.New()
+		driverMock.Failures[mockdriver.ActionTap] = errors.New("tap driver error")
+		action := verifier.Action{Kind: verifier.ActionKindInputText, X: 10, Y: 20, Text: "alice"}
+
+		err := applyAction(context.Background(), driverMock, action, nil)
+		if err == nil {
+			t.Fatalf("expected focus tap failure to surface, got nil")
+		}
+		if containsAction(driverMock.Actions(), mockdriver.ActionInputText, "") {
+			t.Errorf("InputText must not run after focus tap failed: %v", driverMock.Actions())
+		}
+	})
+}
+
 func mustNewVerifier(t *testing.T) *verifier.Verifier {
 	t.Helper()
 	verifierInstance, err := verifier.New()