test(web): gate the web engine at 100% coverage (vitest v8 + CI)

.github/workflows/web.yml

@@ -0,0 +1,33 @@
+name: Web
+
+# Typecheck and run the web test suite with the 100% engine-coverage gate.
+# The threshold lives in web/vite.config.ts, so test:coverage fails the build if
+# any pure-engine statement, branch, function, or line is left uncovered.
+on:
+  push:
+    branches: [main]
+    paths:
+      - 'web/**'
+      - '.github/workflows/web.yml'
+  pull_request:
+    paths:
+      - 'web/**'
+      - '.github/workflows/web.yml'
+
+jobs:
+  test:
+    name: Typecheck and coverage gate
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: web
+    steps:
+      - uses: actions/checkout@v4
+      - uses: actions/setup-node@v4
+        with:
+          node-version: '22'
+          cache: npm
+          cache-dependency-path: web/package-lock.json
+      - run: npm ci
+      - run: npm run typecheck
+      - run: npm run test:coverage

web/package.json

@@ -9,7 +9,8 @@
     "build": "tsc --noEmit && vite build",
     "preview": "vite preview",
     "typecheck": "tsc --noEmit",
-    "test": "vitest run"
+    "test": "vitest run",
+    "test:coverage": "vitest run --coverage"
   },
   "dependencies": {
     "react": "^19.2.7",
@@ -20,6 +21,7 @@
     "@types/react": "^19.2.17",
     "@types/react-dom": "^19.2.3",
     "@vitejs/plugin-react": "^6.0.2",
+    "@vitest/coverage-v8": "^4.1.9",
     "tailwindcss": "^4.3.1",
     "typescript": "^6.0.3",
     "vite": "^8.0.16",

web/src/engine/analytics.ts

@@ -31,6 +31,9 @@ export function power(baselineRate: number, mde: number, nPerArm: number, alpha
   const p1 = baselineRate
   const p2 = Math.min(Math.max(baselineRate + mde, 0), 1)
   const se = Math.sqrt((p1 * (1 - p1)) / nPerArm + (p2 * (1 - p2)) / nPerArm)
+  // Defensive: se is always > 0 here, because the guard above forces baselineRate
+  // into (0,1), so p1(1-p1) > 0. Kept as a safety net; unreachable by construction.
+  /* v8 ignore next */
   if (!(se > 0)) return 0
   const zAlpha = normalQuantile(1 - alpha / 2)
   const z = Math.abs(mde) / se - zAlpha