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

tests/emulator/record_gif.mjs

@@ -12,6 +12,18 @@
 // At stride=2 we end up with a 30 fps GIF that maps 1:1 to every
 // other NES frame (NES runs at ~60 fps), which is the right
 // tradeoff between smoothness and file size for a README demo.
+//
+// IMPORTANT: `docs/platformer.gif` is committed and embedded in the
+// README. Any change to the compiler, the runtime, the harness, or
+// `examples/platformer.ne` that alters the gameplay you see in the
+// first ~6 seconds of the demo must be followed by
+//
+//     node tests/emulator/record_gif.mjs platformer 360 2 docs/platformer.gif
+//
+// committed alongside the source change. The CI `emulator` job
+// regenerates the gif and fails if the committed copy is stale —
+// gifenc + jsnes are deterministic, so the freshly-rendered bytes
+// byte-match a valid commit. See `.github/workflows/ci.yml`.
 
 import { promises as fs } from "node:fs";
 import path from "node:path";