ci: run the parity suite with 2 GUI workers

--gui-workers 2: two fixtures at once, each on its own display. Measured
~1.5 cores peak / ~1.9 GB on the 4-vCPU / 16 GB runner (3+ contends); each
fixture is fully isolated so this stays correctness-neutral. Roughly halves
the GUI-bound wall time.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XqSEqBGY9RYF96J8aCgxGy

.github/workflows/parity.yml

@@ -111,9 +111,13 @@ jobs:
           for f in ${{ github.event.inputs.fixtures || '' }}; do
             fixtures="$fixtures --fixture $f"
           done
+          # 2 concurrent GUI displays: measured ~1.5 cores peak / ~1.9 GB on
+          # this 4-vCPU / 16 GB runner (3+ would contend); each fixture is
+          # fully isolated so this is correctness-neutral
           python3 orca-test-repo/parity/run_parity.py \
             --slicer-root "$ORCA_SLICER_ROOT" --bin "$ORCA_BIN" \
             --cli-presets "${{ github.event.inputs.cli_presets || 'flat' }}" \
+            --gui-workers 2 \
             --out "$PWD/parity-out" $fixtures
 
       - name: Publish job summary