Deploy the web page to Cloudflare Pages

[fontanf]

Stacked on #634, then #633 and #632. This PR's diff only shows the last step.

Workflow (.github/workflows/wasm.yml)

• build job, on pull requests touching wasm/, src/, include/, extern/ or CMakeLists.txt, and before each deployment:
  • installs Emscripten 6.0.11 (mymindstorm/setup-emsdk, cached) and Node.js 22;
  • builds the module (--parallel 4, as the other workflows);
  • runs node --test wasm/tests/;
  • assembles the site: the page, the module, the Web Worker and _headers, without serve.py.
• deploy job: uploads the site to Cloudflare Pages with cloudflare/wrangler-action.
  • It runs every time the Build workflow succeeds on master, like the Python workflow's publishing, so a broken master is never deployed. It can also be run manually on master.
  • It uses the cloudflare-pages environment, so a protection rule can be added there if wanted.
• Same conventions as python.yml: no runs for fork pull requests, and outdated pull request runs are cancelled.

wasm/web/_headers sets the two headers enabling SharedArrayBuffer on Cloudflare Pages. wasm/README.md documents the deployment.

Setup required before the first deployment

1. A Cloudflare account, and a Pages project named packingsolver, created for direct upload: in the dashboard, Workers & Pages → Create → Pages → "Upload assets". Another name can be used by setting the repository variable CLOUDFLARE_PAGES_PROJECT.
2. A Cloudflare API token with the "Cloudflare Pages: Edit" account permission.
3. The repository secrets CLOUDFLARE_API_TOKEN and CLOUDFLARE_ACCOUNT_ID.

Testing

• The site assembled as in the workflow, served as static files with only the _headers rules applied, as Cloudflare Pages does, in headless Chromium:
  • crossOriginIsolated is true;
  • the six problem types solve and draw their solutions;
  • "Stop" works;
  • no console errors.
• The workflow itself hasn't run yet: it runs on pull requests to master, so once this stack is retargeted to master.