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

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.
 

scripts/pre-commit

@@ -39,4 +39,26 @@ if [ $stale -ne 0 ]; then
     exit 1
 fi
 
+# Only rebuild docs/platformer.gif when platformer.nes or the gif
+# recorder itself changed — the gif regeneration takes ~20s due to
+# puppeteer's cold start, so we don't want to pay it on every commit.
+# Skipped entirely if node_modules isn't installed in the emulator
+# harness; the CI `emulator` job is the authoritative check.
+changed_files=$(git diff --cached --name-only)
+if echo "$changed_files" | grep -qE '^(examples/platformer\.(ne|nes)|tests/emulator/record_gif\.mjs|tests/emulator/harness\.html)$'; then
+    if [ -d tests/emulator/node_modules ]; then
+        echo "  Rebuilding docs/platformer.gif (platformer.nes or recorder changed)..."
+        (cd tests/emulator && node record_gif.mjs platformer 360 2 /tmp/platformer-precheck.gif >/dev/null)
+        if ! cmp -s docs/platformer.gif /tmp/platformer-precheck.gif; then
+            echo "    STALE: docs/platformer.gif"
+            echo "           rerun: node tests/emulator/record_gif.mjs platformer 360 2 docs/platformer.gif"
+            rm -f /tmp/platformer-precheck.gif
+            exit 1
+        fi
+        rm -f /tmp/platformer-precheck.gif
+    else
+        echo "  (skipping gif freshness check — tests/emulator/node_modules not installed)"
+    fi
+fi
+
 echo "All pre-commit checks passed."

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";