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.

examples/sample-app/README.md

@@ -47,4 +47,4 @@ Traces land in `./runs/<timestamp>/`.
   registers snapshot extractors (`app_state`, `click_count`)
 - `spec.ts` imports `@uatu/spec` (see `package.json`), reads those snapshots,
   asserts properties on them, and weights the actions the fuzzer picks from
-- `just test` invokes `uatu test` against the installed APK on the named AVD
+- `just test` invokes `uatu test` against the installed APK on the connected device (or the AVD named via `AVD=`)