docs: regenerate platformer.gif and lock it as a CI invariant

The optimizer fix in the previous commit changes the observable
gameplay of `examples/platformer.ne` — pre-fix the player got
spurious enemy-1 stomp bounces every time coin 2 drifted into its
pickup window, so the README demo gif showed the player bouncing
mid-air around emu frames 85-125 instead of walking through the
coin at ground level. Regenerate `docs/platformer.gif` from the
fixed compiler so the README matches reality.

To stop this from drifting again, treat the gif the same way the
repo already treats `examples/*.nes`:

- `gifenc` + `jsnes` + the harness are deterministic, so a fresh
  recording byte-matches a valid commit. Verified across two
  back-to-back runs (identical md5).

- `.github/workflows/ci.yml`'s `emulator` job now renders the gif
  into `/tmp/platformer.gif` and `cmp`s it against `docs/platformer.gif`,
  emitting a `::error` annotation pointing at the exact rerun
  command if the committed copy is stale. This piggybacks on the
  existing puppeteer + node setup, adding ~20s to the job.

- `scripts/pre-commit` runs the same check locally, but only when
  `examples/platformer.{ne,nes}`, `tests/emulator/record_gif.mjs`,
  or `tests/emulator/harness.html` is staged, and only if
  `tests/emulator/node_modules` is already installed. Cold-start
  puppeteer is ~20s — too slow to pay on every commit, but cheap
  enough to pay when something gif-relevant changed.

- The header of `tests/emulator/record_gif.mjs` and the project
  conventions section of `CLAUDE.md` both spell out the rerun
  command and the invariant, so the next agent doesn't have to
  re-derive any of this.

https://claude.ai/code/session_013Bi4H4YQ5or5HtMB4doUFi

.github/workflows/ci.yml

@@ -146,6 +146,21 @@ jobs:
       - name: Diff each ROM's framebuffer against its golden
         working-directory: tests/emulator
         run: node run_examples.mjs
+      - name: Verify docs/platformer.gif is up to date
+        # The README embeds docs/platformer.gif as the project demo.
+        # gifenc + jsnes are deterministic, so rebuilding the gif from
+        # the current compiler should byte-match the committed copy.
+        # If it doesn't, the contributor changed the compiler / source
+        # / harness in a way that affects the observable gameplay of
+        # platformer.ne but forgot to regenerate the gif — the fix is
+        # the exact command printed in the error.
+        working-directory: tests/emulator
+        run: |
+          node record_gif.mjs platformer 360 2 /tmp/platformer.gif
+          if ! cmp -s ../../docs/platformer.gif /tmp/platformer.gif; then
+            echo "::error file=docs/platformer.gif::committed docs/platformer.gif is stale; rerun \`node tests/emulator/record_gif.mjs platformer 360 2 docs/platformer.gif\` and commit the new gif"
+            exit 1
+          fi
       - name: Upload actual + diff PNGs on failure
         if: failure()
         uses: actions/upload-artifact@v4