release: OpenScene 0.3.1

[sjungwon03]

Summary

Promotes vetted dev to main for the OpenScene 0.3.1 release. This promotion triggers the version-gated release workflow exactly once.

Validation

• dev PR CI passed: typecheck, test, and build.
• The release workflow gates on the absence of tag v0.3.1.
• Store distribution is restricted to the main release workflow and protected main environment.

Checklist

• This pull request targets main.
• The release version was bumped on dev before promotion.
• The source commit passed CI and review.
• App Store Connect and Google Play credentials remain GitHub Environment Secrets.

Notes for reviewers

After merge, the release workflow will create tag v0.3.1, publish desktop artifacts, upload iOS build 0.3.1 (2) to App Store Connect, and publish Android versionCode 2 to the Play production track.

[sjungwon03-ai]

This PR prepares the 0.3.1 release by adding production-grade Android and iOS release workflows, updating RELEASING.md and configs, and adjusting tests. CI checks (release-promotion, verify) are green on head 7381bb8. Changes use secure secret patterns, pinned actions, and proper Expo/Android build steps with no code-quality issues, security concerns, or test regressions found.