feat(driver): Driver interface for v0.1 RPC surface

Mirrors proto/driverpb/driver.proto: Launch, Terminate, Tap,
InputText, Hierarchy, Screenshot, WaitForIdle, Health. Keeps
Image/Health as dedicated structs so callers don't depend on
generated proto types.

internal/driver/driver.go

@@ -0,0 +1,35 @@
+package driver
+
+import (
+	"context"
+	"time"
+)
+
+// Driver abstracts the platform-specific UI automation backend. v0.1 surface
+// matches proto/driverpb/driver.proto. The Maestro sidecar implementation
+// lives under driver/maestro; tests use driver/mock.
+type Driver interface {
+	Launch(ctx context.Context, bundleID string, clearState bool) error
+	Terminate(ctx context.Context) error
+
+	Tap(ctx context.Context, x, y int) error
+	InputText(ctx context.Context, text string) error
+
+	Hierarchy(ctx context.Context) (string, error)
+	Screenshot(ctx context.Context) (Image, error)
+
+	WaitForIdle(ctx context.Context, duration time.Duration) error
+	Health(ctx context.Context) (Health, error)
+}
+
+type Image struct {
+	PNG    []byte
+	Width  int
+	Height int
+}
+
+type Health struct {
+	Ready    bool
+	Version  string
+	Platform string
+}