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.

nix/checked-candidate.nix

@@ -65,6 +65,27 @@ let
       }) (flatten root)
     )
   );
+  # Frontends are an explicit build choice. The capability assembler itself
+  # neither discovers React contracts nor loads executable package code.
+  queryFrontends = pkgs.lib.unique (
+    pkgs.lib.concatMap (
+      node:
+      let
+        config = node.directory + "/quixos.check.json";
+      in
+      if builtins.pathExists config then
+        (builtins.fromJSON (builtins.readFile config)).queryFrontends or [ ]
+      else
+        [ ]
+    ) (capabilityFlatten root)
+  );
+  queryGeneratorFlags =
+    if queryFrontends == [ ] then
+      ""
+    else if queryFrontends == [ "react" ] then
+      "--query-generator ${protocol}/libexec/quixos-protocol/react-query-generator.mjs"
+    else
+      throw "Unsupported query frontend selection: ${builtins.toJSON queryFrontends}";
   snapshots = pkgs.writeText "qx-nix-source-graph.json" (
     builtins.toJSON {
       resources = map (node: {
@@ -85,6 +106,7 @@ let
         if node.kind == "workspace" then
           ''
             ${protocol}/bin/quixos-workspace-compile --root ${node.directory} \
+              ${queryGeneratorFlags} \
               --source-root-commit ${pkgs.lib.escapeShellArg node.commit} \
               --checkout-root "$TMPDIR/checkouts" --snapshot-map ${snapshots} \
               --graph-out "$out/graph.json" --schemas-out "$out/package-schemas.json" \
@@ -93,6 +115,7 @@ let
         else
           ''
             ${protocol}/bin/quixos-resource-compile --root ${node.directory} \
+              ${queryGeneratorFlags} \
               --kind ${node.kind} --repository ${pkgs.lib.escapeShellArg node.repository} \
               --commit ${pkgs.lib.escapeShellArg node.commit} \
               --checkout-root "$TMPDIR/checkouts" --snapshot-map ${snapshots} --snapshot-only true \