Add checked query composition foundations for prepared presentations

Move React navigation query generation behind an explicit build frontend. Add checked result-dependent query bindings and pure composition evaluation, including one-snapshot server execution. Preserve the existing rendering path while the React manifest, worker preparation, and replica admission integration remain pending; see docs/PREPARED_PRESENTATIONS_PROGRESS.md.

src/capability-language/query-generator.ts

@@ -0,0 +1,13 @@
+import path from "node:path";
+import { pathToFileURL } from "node:url";
+import type { PackageQueryGenerator } from "./assembly.js";
+
+/** The build invoker selects this executable explicitly. Never resolve a
+ * generator by a capability name or execute a module discovered in user QX. */
+export async function loadQueryGenerator(filename?: string): Promise<PackageQueryGenerator | undefined> {
+  if (!filename) return undefined;
+  const module = await import(pathToFileURL(path.resolve(filename)).href);
+  if (typeof module.generatePackageQueries !== "function")
+    throw new Error("Query generator must export generatePackageQueries");
+  return module.generatePackageQueries;
+}