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/__tests__/client-route-zod-guard.test.ts

@@ -1,5 +1,6 @@
 /**
- * The documents vault and the Coach load no schema library in the browser.
+ * The documents vault, the Coach and the metric pages load no schema library
+ * in the browser.
  *
  * Both routes reached zod through a constant: the list of document kinds, the
  * bulk limit, the attachment cap, the Coach data clusters, a conversation
@@ -121,6 +122,11 @@ describe("client routes without the schema library", () => {
     "src/app/documents/page.tsx",
     "src/app/coach/page.tsx",
     "src/app/coach/conversations/page.tsx",
+    // The metric sub-pages read the plausibility band through the value
+    // domain (`src/lib/measurements/value-ranges.ts`), and the values list
+    // checks its segment against the type label map.
+    "src/app/insights/pulse/page.tsx",
+    "src/app/insights/values/[type]/page.tsx",
   ]) {
     it(`${route} ships no zod in its static client graph`, () => {
       const hits = clientZodImports(route);