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

.gitignore

@@ -7,6 +7,11 @@ public/assets/
 # the git pull at the top of every build tick
 notebooks-export/
 public/nb/
+# the ACannons browser build, staged by cannons_reloaded's export-cannons.sh:
+# same kind of artifact, same reason. It is also ~9MB of generated JavaScript
+# and WebAssembly, which has no business in this repo's history.
+cannons-export/
+public/cn/
 # the reading map is the same kind of artifact: library-rag-export.service regenerates it on
 # cyrion whenever the corpus or its located highlights move. It was tracked until 2026-08-14,
 # which is why automating the export needed this line first.