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

scripts/pre-commit

@@ -86,4 +86,9 @@ if [ $recorder_or_harness_changed -eq 1 ] || \
     check_gif war 4 || exit 1
 fi
 
+if [ $recorder_or_harness_changed -eq 1 ] || \
+   echo "$changed_files" | grep -qE '^examples/pong(\.(ne|nes)|/[^/]+\.ne)$'; then
+    check_gif pong 4 || exit 1
+fi
+
 echo "All pre-commit checks passed."