A3c: Run unit tests + trace corpus in CI

[AsafMah]

Part of #13. Closes the gap in #12.

CI currently only runs compileOfflineRunTestsKotlin (compile-only). Add a CI step that runs :app:testOfflineDebugUnitTest plus the A3 corpus (non-blocking at first), so regressions surface.

Acceptance: PRs run unit tests + corpus; results visible in checks.

[AsafMah]

Already satisfied by #57: build-test-auto.yml runs :app:testOfflineRunTestsUnitTest as a hard gate on every PR to dev/main + pushes to dev. That suite includes the #21 backspace corpus, #77 BackspaceUnitStackTest, SuggestTest, etc. The runTests build type intentionally self-skips the ~5 network/dictionary/emoji/locale-flaky tests (tracked under #12). Closing as done. The remaining real gap — native gesture-recognition coverage — is split out into its own issue.