feat: enforce web vitals performance budgets

tests/unit/performance-budgets.test.js

@@ -0,0 +1,77 @@
+import { describe, expect, it } from "vitest";
+
+import {
+  evaluateBundleBudget,
+  evaluateFieldBudget,
+  evaluateLabBudget,
+  percentile75,
+} from "../../src/application/policies/performance-budgets.js";
+
+describe("performance budgets", () => {
+  it("rejects initial and lazy JavaScript above their named limits", () => {
+    const thresholds = {
+      initialJsGzipBytes: 200,
+      lazyChunkGzipBytes: 120,
+    };
+    expect(
+      evaluateBundleBudget(
+        { initialJsGzipBytes: 201, lazyChunks: [] },
+        thresholds,
+      ).passed,
+    ).toBe(false);
+    expect(
+      evaluateBundleBudget(
+        {
+          initialJsGzipBytes: 100,
+          lazyChunks: [{ path: "lazy.js", gzipBytes: 121 }],
+        },
+        thresholds,
+      ).passed,
+    ).toBe(false);
+  });
+
+  it("fails lab evidence when context is absent or a metric is over budget", () => {
+    const thresholds = { lcpMs: 2500, cls: 0.1, namedInteractionMs: 200 };
+    expect(
+      evaluateLabBudget(
+        { metrics: { lcpMs: 1000, cls: 0, namedInteractionMs: 50 } },
+        thresholds,
+      ),
+    ).toMatchObject({ passed: false });
+    expect(
+      evaluateLabBudget(
+        {
+          context: {
+            runner: {},
+            browser: {},
+            viewport: {},
+            network: {},
+            cpu: {},
+            cache: {},
+            build: {},
+          },
+          metrics: { lcpMs: 2501, cls: 0, namedInteractionMs: 50 },
+        },
+        thresholds,
+      ).passed,
+    ).toBe(false);
+  });
+
+  it("uses the nearest-rank p75 and fails closed while sample minimum is deferred", () => {
+    expect(percentile75([4, 1, 3, 2])).toBe(3);
+    expect(
+      evaluateFieldBudget(
+        {
+          metrics: { p75LcpMs: 1000, p75Cls: 0.01, p75InpMs: 50 },
+          eligibleSamples: 100,
+        },
+        {
+          p75LcpMs: 2500,
+          p75Cls: 0.1,
+          p75InpMs: 200,
+          minimumEligibleSamples: null,
+        },
+      ),
+    ).toEqual({ status: "FAIL_UNVERIFIED", passed: false });
+  });
+});