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

CLAUDE.md

@@ -22,6 +22,22 @@ re-derive the project conventions from scratch.
   tmp path and fails if the committed version differs, pointing at
   the exact `cargo run -- build examples/<name>.ne` to run. The
   pre-commit hook under `scripts/pre-commit` catches this locally.
+- **`docs/platformer.gif` is committed** and embedded in the
+  top-level README as the project demo. `gifenc` + `jsnes` are
+  deterministic, so the gif's bytes are a function of the compiler,
+  the runtime, the harness, and `examples/platformer.ne`. Any change
+  to those that affects the first ~6 seconds of observable platformer
+  gameplay must be followed by regenerating the gif:
+
+  ```bash
+  node tests/emulator/record_gif.mjs platformer 360 2 docs/platformer.gif
+  ```
+
+  and committing it in the same change. CI's `emulator` job renders
+  a fresh gif and fails if the committed one doesn't byte-match. The
+  pre-commit hook rebuilds the gif when `platformer.ne`, `platformer.nes`,
+  `record_gif.mjs`, or `harness.html` is staged (and `tests/emulator/node_modules`
+  is installed).
 - `docs/future-work.md` lists the remaining gaps. If you implement
   something from that file, update the doc in the same PR.