refactor: validate generated evidence artifacts

scripts/collect-web-vitals-evidence.ts

@@ -1,10 +1,12 @@
-import { mkdir, readFile, writeFile } from "node:fs/promises";
+import { mkdir, readFile } from "node:fs/promises";
 
 import {
   evaluateFieldBudget,
   percentile75,
 } from "../src/application/policies/performance-budgets.ts";
+import { fieldWebVitalsArtifactSchema } from "./contracts/release-artifacts.ts";
 import { validateFieldEvidenceInput } from "./lib/field-vitals-evidence.ts";
+import { writeValidatedJsonArtifact } from "./lib/validated-json-artifact.ts";
 
 const inputPath =
   process.env.FIELD_WEB_VITALS_INPUT ||
@@ -94,10 +96,11 @@ const report = {
 };
 
 await mkdir("artifacts/performance", { recursive: true });
-await writeFile(
-  "artifacts/performance/field-web-vitals.json",
-  `${JSON.stringify(report, null, 2)}\n`,
-);
+await writeValidatedJsonArtifact({
+  path: "artifacts/performance/field-web-vitals.json",
+  schema: fieldWebVitalsArtifactSchema,
+  value: report,
+});
 if (!passed) {
   process.stderr.write(
     `Field Web Vitals: ${status} (approved threshold decision and valid 28-day production evidence are required)\n`,