perf(insights): keep Zod off the first load of the metric pages

The value domain read the plausibility band out of the measurement
validation module, so every metric sub-page that asks whether a metric
can be negative or is integer-valued loaded the measurement schemas and
Zod with them. The band and its check live in a Zod-free module now,
re-exported by the validation module for the server writers, and the
values list checks its segment against the exhaustive type label map
instead of the Zod enum. About fifty metric routes drop roughly 88 KB
gzip from their first load. The client-route guard walks a metric page
and the values list too.

src/app/insights/values/[type]/page.tsx

@@ -8,8 +8,8 @@ import {
   INSIGHTS_OVERVIEW,
   resolveValuesBackHref,
 } from "@/lib/insights/values-back-link";
-import { measurementTypeEnum } from "@/lib/validations/measurement";
 import { MEASUREMENT_TYPE_LABEL_KEYS } from "@/components/measurements/measurement-list-meta";
+import { MEASUREMENT_TYPE_LABEL_KEYS as EVERY_MEASUREMENT_TYPE } from "@/lib/measurements/type-label-keys";
 import { BackLink } from "@/components/ui/back-link";
 import { MeasurementList } from "@/components/measurements/measurement-list";
 import { SubPageShell } from "@/components/insights/sub-page-shell";
@@ -38,8 +38,11 @@ export default function InsightsMetricValuesPage({
   const searchParams = useSearchParams();
 
   // Reject any segment that is not a real MeasurementType so a typo'd
-  // deep-link 404s instead of mounting a blank list.
-  if (!measurementTypeEnum.options.includes(type as never)) {
+  // deep-link 404s instead of mounting a blank list. The leaf label map is
+  // typed `Record<MeasurementType, string>`, so the compiler holds it to
+  // exactly the enum, and it answers the question without pulling the
+  // measurement schemas, and Zod, into the page.
+  if (!Object.hasOwn(EVERY_MEASUREMENT_TYPE, type)) {
     notFound();
   }