v2: Test runner integration

[loyality7]

Context

Lets web UI tests run from an Android instrumentation test directly, without a desktop harness driving the device.

Scope

• A pattern (and ideally a small helper library) for using WebDroid inside androidx.test instrumented tests: setup/teardown, waiting for the library's own coroutines from a JUnit test thread.

Acceptance criteria

• A documented, working example: an instrumented test that drives a page and asserts on it, no desktop machine involved
• Teardown is reliable — no leaked sessions across test runs (same discipline as the library's own instrumented suite)