docs: add docs/pong.gif demo to README

Record a 6-second gif of examples/pong.nes running in jsnes and
embed it alongside docs/platformer.gif and docs/war.gif as the
third project demo. The gif opens on Pong's title menu (CPU VS
CPU / 1 PLAYER / 2 PLAYERS) — warmup = 4 frames keeps the menu
as the thumbnail the way war's recording does, and then the
headless autopilot advances to gameplay partway through the
clip.

- docs/pong.gif committed (128 KB)
- README.md links it under the war demo
- scripts/pre-commit rebuilds it when examples/pong* or the
  recorder/harness change
- .github/workflows/ci.yml fails if the committed copy is stale
- CLAUDE.md and tests/emulator/record_gif.mjs reference the new
  gif in the "how to regenerate" sections

https://claude.ai/code/session_0134F5uwDEVTes2Ee9S7JeXy

.github/workflows/ci.yml

@@ -176,6 +176,19 @@ jobs:
             echo "::error file=docs/war.gif::committed docs/war.gif is stale; rerun \`node tests/emulator/record_gif.mjs war 360 2 docs/war.gif 4\` and commit the new gif"
             exit 1
           fi
+      - name: Verify docs/pong.gif is up to date
+        # Same contract as the platformer and war gifs: the README
+        # embeds docs/pong.gif as the third project demo. Pong's
+        # title screen (CPU VS CPU / 1 PLAYER / 2 PLAYERS menu) is
+        # the gif thumbnail, so warmup = 4 keeps it in frame before
+        # the autopilot advances to gameplay.
+        working-directory: tests/emulator
+        run: |
+          node record_gif.mjs pong 360 2 /tmp/pong.gif 4
+          if ! cmp -s ../../docs/pong.gif /tmp/pong.gif; then
+            echo "::error file=docs/pong.gif::committed docs/pong.gif is stale; rerun \`node tests/emulator/record_gif.mjs pong 360 2 docs/pong.gif 4\` and commit the new gif"
+            exit 1
+          fi
       - name: Upload actual + diff PNGs on failure
         if: failure()
         uses: actions/upload-artifact@v4