Stage the ACannons browser build at /cannons

A third instance of the pattern notebooks-export/ and the reading payloads
already use: an artifact built somewhere with a toolchain this repo's
node:24-slim container does not have, dropped into a gitignored directory,
staged into public/ by a script, and picked up by auto-build.sh's signature on
the next tick. Nothing here needs a JDK, Go, or an opinion about the game --
cannons_reloaded's deploy/export-cannons.sh hands over a directory that is
already ready to serve.

Three things are load-bearing and easy to get wrong later:

The payload is at /cn/, not /cannons/. With build.format:'file' the page
builds to cannons.html, and try_files would then have a file and a directory
competing for the same URL. That is exactly why the notebook exports live at
/nb/ and not under /notebooks/.

nginx gets gzip_static for /cn/. nginx:alpine has `gzip on` commented out in
its base config, so without it the page ships 9MB instead of 3.3MB and nothing
anywhere reports that. It gets no-cache rather than an expiry, because
classes.js and cannons.wasm are stable filenames whose contents change on
every rebuild -- a long expiry would pair a stale client with a fresh server.

The page loads on click, not `loading="lazy"`. Lazy only defers until the
frame nears the viewport and this frame is above the fold, so a few MB would
download for anyone who opened the page to read the paragraph.

A missing export is a normal state throughout: copy-cannons.mjs stages
nothing, the page says so, and the sidebar link does not appear.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S4wdbGSVzvUDHBWRjSVxfr

package.json

@@ -5,7 +5,7 @@
   "private": true,
   "scripts": {
     "dev": "astro dev",
-    "build": "node scripts/copy-assets.mjs && node scripts/copy-notebooks.mjs && astro build",
+    "build": "node scripts/copy-assets.mjs && node scripts/copy-notebooks.mjs && node scripts/copy-cannons.mjs && astro build",
     "preview": "astro preview"
   },
   "dependencies": {