feat: enforce web vitals performance budgets

docs/operations/performance-evidence.md

@@ -0,0 +1,17 @@
+# Performance evidence contract
+
+Performance evidence is deliberately split by measurement context:
+
+- `bundle.json` records production build output and enforces initial JavaScript
+  at 200 KiB gzip and every lazy chunk at 120 KiB gzip.
+- `lab.json` records Chromium/runner/viewport/network/CPU/cache/build context and
+  enforces LCP 2.5 s, CLS 0.10, and the named route interaction at 200 ms.
+- `field-web-vitals.json` records consent-filtered, route-ID aggregated,
+  release-specific production samples over 28 days and evaluates p75 LCP, CLS,
+  and INP against 2.5 s, 0.10, and 200 ms.
+
+The field minimum eligible-sample threshold is intentionally unresolved until
+a privacy-approved telemetry baseline exists. Therefore the field command
+fails closed with `FAIL_UNVERIFIED` when run against the example input. Provide
+`FIELD_WEB_VITALS_INPUT` and a reviewed `MIN_ELIGIBLE_SAMPLES` only after that
+decision is recorded.