Enforce a performance budget in CI

Add the gate

1. Step 01

   Write the budget down

   Assertions live in a config file next to the workflow, so changing the budget shows up in review as a diff rather than a discussion.

   {
     "ci": {
       "collect": {
         "url": [
           "http://localhost:3000/",
           "http://localhost:3000/blog/seo-nextjs-app-router",
           "http://localhost:3000/docs/server-setups/provision-ubuntu-node-server"
         ],
         "numberOfRuns": 3,
         "settings": { "preset": "desktop" }
       },
       "assert": {
         "assertions": {
           "categories:performance": ["error", { "minScore": 0.9 }],
           "categories:seo": ["error", { "minScore": 1 }],
           "categories:accessibility": ["error", { "minScore": 0.95 }],
           "largest-contentful-paint": ["error", { "maxNumericValue": 2500 }],
           "cumulative-layout-shift": ["error", { "maxNumericValue": 0.1 }]
         }
       }
     }
   }

2. Step 02

   Run it against a production build

   Never audit the dev server. It ships unminified bundles and no caching, so the numbers are meaningless and the gate will either always fail or be set so loose it never fires.

   - run: npm ci
   - run: npm run build
   - run: npx --yes @lhci/cli autorun --config=lighthouserc.json

3. Step 03

   Median of three runs, not one

   A single Lighthouse run on shared CI hardware varies by ten points or more. Three runs with the median taken is the smallest number that produces a gate people trust rather than retry.