fix(runner): warn on malformed screen snapshot (#13)

* fix(runner): warn on malformed screen snapshot

screenFromSnapshot swallowed json.Unmarshal errors, so a non-string
screen value silently became "" in the step log and trace while the
verifier still saw the raw JSON. Return the error and warn at the
call site, matching the hierarchy warning pattern.

* docs: clarify --avd is optional for uatu test

The CLI accepts --avd as an empty-string default (cmd/uatu/main.go:49)
and only requires it when no device is connected and multiple AVDs
exist (cmd/uatu/android_env.go:63). Docs and examples that showed it
as required or always-passed were misleading.

docs/manual/runs.md

@@ -11,7 +11,7 @@ A run is not analogous to a unit test. A closer framing is: boot a fuzzer for an
 ## Lifecycle
 
 ```
-uatu test --spec spec.ts --bundle-id com.example.app --avd Pixel_7 --duration 30m
+uatu test --spec spec.ts --bundle-id com.example.app --duration 30m
   │
   ├── uninstall and reinstall the app (clean slate, every run)
   ├── boot the sidecar, connect the agent socket