Add persistent virtual display for all UI regression tests

The browser find focus test was failing because XCUIApplication.launch()
cannot foreground-activate the app on headless CI runners (WarpBuild)
without a display. The display resolution test already had its own virtual
display, but it was scoped to that step only.

Create a persistent virtual display at the start of the ui-regressions job
that stays alive for all test steps. Also switch the browser test from
`test` to `test-without-building` since the build step already ran.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -488,6 +488,36 @@ jobs:
             -destination "platform=macOS" \
             build-for-testing
 
+      - name: Create persistent virtual display
+        run: |
+          set -euo pipefail
+          HELPER_PATH="/tmp/create-virtual-display"
+          clang -framework Foundation -framework CoreGraphics \
+            -o "$HELPER_PATH" scripts/create-virtual-display.m
+
+          VDISPLAY_READY="/tmp/cmux-vdisplay-persistent.ready"
+          VDISPLAY_ID_PATH="/tmp/cmux-vdisplay-persistent.id"
+          rm -f "$VDISPLAY_READY" "$VDISPLAY_ID_PATH"
+
+          "$HELPER_PATH" \
+            --modes "1920x1080" \
+            --ready-path "$VDISPLAY_READY" \
+            --display-id-path "$VDISPLAY_ID_PATH" \
+            > /tmp/cmux-vdisplay-persistent.log 2>&1 &
+          echo "VDISPLAY_PERSISTENT_PID=$!" >> "$GITHUB_ENV"
+
+          echo "Waiting for persistent virtual display..."
+          for i in $(seq 1 24); do
+            if [ -f "$VDISPLAY_READY" ]; then break; fi
+            sleep 0.5
+          done
+          if [ ! -f "$VDISPLAY_READY" ]; then
+            echo "ERROR: Persistent virtual display not ready after 12s" >&2
+            cat /tmp/cmux-vdisplay-persistent.log 2>/dev/null || true
+            exit 1
+          fi
+          echo "Persistent virtual display ready: ID=$(cat "$VDISPLAY_ID_PATH")"
+
       - name: Run display resolution churn UI regression
         run: |
           set -euo pipefail
@@ -661,4 +691,12 @@ jobs:
             -destination "platform=macOS" \
             -maximum-test-execution-time-allowance 120 \
             -only-testing:cmuxUITests/BrowserPaneNavigationKeybindUITests/testCmdFFocusesBrowserFindFieldAfterCmdDCmdLNavigation \
-            test
+            test-without-building
+
+      - name: Cleanup persistent virtual display
+        if: always()
+        run: |
+          if [ -n "${VDISPLAY_PERSISTENT_PID:-}" ]; then
+            kill "$VDISPLAY_PERSISTENT_PID" >/dev/null 2>&1 || true
+          fi
+          rm -f /tmp/cmux-vdisplay-persistent.ready /tmp/cmux-vdisplay-persistent.id /tmp/cmux-vdisplay-persistent.log