docs: trpc now optional — prerequisite notes + conditional HTML

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docs/architecture/data-flow-explainer.html

@@ -2801,7 +2801,7 @@ function makeStages(F) {
       meta: `@repo/${F.name}/ui`,
       file: `packages/${F.name}/src/ui/query.ts`,
       tag: "client",
-      prose: `<code>${F.queryName}</code> on the client builds a typed React Query option object. The args type is inferred all the way back from the use case's <code>${F.inputSchemaName}</code>, exported from the feature root.`,
+      prose: `<code>${F.queryName}</code> on the client builds a typed React Query option object. The args type is inferred all the way back from the use case's <code>${F.inputSchemaName}</code>, exported from the feature root. <em>Note: <code>useTRPC()</code> is provided by <code>@repo/core-trpc</code> — an optional package. Scaffold it with <code>pnpm turbo gen core-package trpc</code> if not yet present.</em>`,
       code: F.queryName.startsWith("(") ?
 `// auth procedures are mutations — invoked via useMutation, not useQuery
 const trpc = useTRPC();