ci(examples): add the replay-ui fixture action

Records a trace and serves it with sanderling replay. The step page URL
is a composite output rather than GITHUB_ENV, so it is scoped to the one
step that drives it.

.github/actions/replay-ui-fixture/action.yml

@@ -0,0 +1,55 @@
+name: replay ui fixture
+description: Record a trace with sanderling, then serve it with sanderling replay.
+
+outputs:
+  url:
+    description: the step page of the served run, for a spec to drive
+    value: ${{ steps.serve.outputs.url }}
+
+runs:
+  using: composite
+  steps:
+    # A trace with a violation and uncaught exceptions in it, so the UI has
+    # something to render in every panel the spec looks at. No
+    # --exit-on-violation here: the run is the fixture, and stopping it at the
+    # first violation would leave a four-step trace to fuzz.
+    - name: Record a fixture trace
+      shell: bash
+      run: |
+        python3 -m http.server 8792 --bind 127.0.0.1 \
+          --directory test/browser/testdata/throwing &
+        ready=""
+        for _ in $(seq 1 30); do
+          curl -sf http://127.0.0.1:8792/ >/dev/null && { ready=1; break; }
+          sleep 1
+        done
+        if [ -z "$ready" ]; then
+          echo "the fixture http server never answered on 127.0.0.1:8792" >&2
+          exit 1
+        fi
+        ./bin/sanderling test \
+          --platform web \
+          --spec test/browser/testdata/throwing/spec.ts \
+          --bundle-id http://127.0.0.1:8792/ \
+          --duration 5m --max-steps 25 --seed 7 \
+          --output runs/fixture
+
+    - name: Serve the trace with sanderling replay
+      id: serve
+      shell: bash
+      run: |
+        # Flags before the positional argument: Go's flag package stops
+        # parsing at the first non-flag word.
+        ./bin/sanderling replay --port 8793 --no-open runs/fixture &
+        ready=""
+        for _ in $(seq 1 30); do
+          curl -sf http://127.0.0.1:8793/api/runs >/dev/null && { ready=1; break; }
+          sleep 1
+        done
+        if [ -z "$ready" ]; then
+          echo "sanderling replay never served /api/runs on 127.0.0.1:8793" >&2
+          exit 1
+        fi
+        run_id="$(ls runs/fixture | head -1)"
+        echo "url=http://127.0.0.1:8793/runs/$run_id/steps/1" >> "$GITHUB_OUTPUT"
+        curl -sf "http://127.0.0.1:8793/runs/$run_id/steps/1" >/dev/null