Stabilize Roborazzi baseline output paths and snapshot workflow docs

[jwoglom]

Motivation

• Keep Roborazzi golden images in deterministic, versioned locations so snapshot baselines can be tracked in the repo and updated intentionally.
• Ensure snapshot preview tests run only when the Roborazzi recording tasks are invoked, while normal unit-test runs remain unaffected.
• Make the wear module behave consistently in CI/offline environments with Robolectric offline properties.

Description

• Update mobile/build.gradle to run snapshot-only tests for Roborazzi tasks by detecting task names and to include snapshot patterns when recording (include '**/snapshots/**') while excluding them for normal runs.
• Update wear/build.gradle with the same Roborazzi task gating and add CI Robolectric offline system properties (robolectric.offline and robolectric.dependency.dir) so CI/offline runs can use a pre-populated Robolectric cache.
• Change mobile and wear snapshot helpers (ComposePreviewSnapshotTests.kt) to write deterministic baseline PNGs under src/test/snapshots/roborazzi/ComposePreviewSnapshotTests/<testName>.png so golden images are stable and commit-able.
• Add .gitignore rules to ignore Roborazzi diff/actual/compare artifacts while allowing the src/test/snapshots baselines to be tracked.
• Document the baseline recording command and the update policy in README.md (how to run ./gradlew :mobile:recordRoborazziDebug :wear:recordRoborazziDebug and commit updated baselines with UI changes).

Testing

• Ran ./gradlew clean :mobile:recordRoborazziDebug :wear:recordRoborazziDebug from a clean environment to validate the configuration and task gating, but snapshot recording did not complete because Robolectric attempted to fetch instrumented Android artifacts and the container environment reported Network is unreachable.
• The Robolectric artifact fetch failure caused Roborazzi snapshot tests to fail (see wear test report failure for ComposePreviewSnapshotTests due to Failed to fetch maven artifact org.robolectric:android-all-instrumented), so no golden PNGs were produced in this environment.
• The changes to Gradle/test helpers and .gitignore were applied and validated syntactically; when run in a network-enabled or Robolectric-offline-prepared environment the recording command will emit baselines to the newly-configured tracked paths.

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Codex Task