test: replace wall-clock loop deadlines with progress criteria

Three real-engine throughput loops (thumbnail pipeline, full-res fill
behind the proxy, playback window supply) failed on machine slowness:
their pass/fail was a wall-clock Instant deadline, so a loaded machine
broke them for speed, not for a broken pipeline. Each loop now counts
engine pumps — machine-speed independent — and asserts the condition
within a bounded number of pumps. The two single-frame worker channel
receives keep a generous 60 s recv_timeout (a one-shot bounded
operation, not a throughput loop).

oak-cli: the integration fixtures moved with the app crate during the
workspace restructure; point the fixture helpers at
../oak-app/tests instead of the (now empty) repo-root tests/.

.gitea/workflows/ci.yml

@@ -184,7 +184,7 @@ jobs:
     runs-on: oak-windows-2025
     defaults:
       run:
-        shell: C:\msys2\usr\bin\bash.exe -eo pipefail {0}
+        shell: msys2 {0}
     steps:
       - name: Checkout
         uses: actions/checkout@v4