Instrument and improve incremental workspace checks

Profile source capture, Nix evaluation/realization, resource/query checking,
and implementation builds without putting timings into immutable outputs.
Keep modular repositories; bound independent capture work, snapshot jj once
per observation, and retain final concurrent-edit and Git completeness checks.

Compose exact resource/closure/frontend/toolchain checks as memoized Nix
derivations. Preserve whole-workspace validation and the full compiler path
when no checked dependency is supplied. Share canonical Nixpkgs by content,
index workspace build contexts once, and narrow lock/driver/compiler inputs
with conservative complete-source fallback. Batch React witness checks only
when distinct source closures cannot share ambient declarations; otherwise
run the original independent TypeScript checks.

Seed local authoring toolchains with exact canonical fetch identities when
they match the selected inputs; retain store-path inputs for overrides.
Add generic correctness checks rather than permanent Reader timing/content
fixtures. Verify actual Reader edits and rejection/recovery in a private
workbench, preserving the original user's workbench and runtime.

Query checking currently caches at resource/closure boundaries, not individual
query semantic dependency boundaries. Template helper pins and reproducible
fixture snapshots follow in a separate publication change.

quixos-package-helpers.nix

@@ -847,6 +847,40 @@ let
 in
 {
   mkCaminoReactPackage = import ./react-package.nix;
+  # Share the composition's package set only for the same exact Nixpkgs source,
+  # system and default import arguments. Config/overlay/cross-build choices use
+  # ordinary Nixpkgs evaluation. No package implementation is substituted.
+  packageSet =
+    args@{
+      nixpkgs,
+      system,
+      context ? { },
+      ...
+    }:
+    let
+      authoredArgs = builtins.removeAttrs args [
+        "nixpkgs"
+        "context"
+      ];
+      importArgs = {
+        config = { };
+        overlays = [ ];
+      }
+      // authoredArgs;
+      environment = context.buildEnvironment or null;
+    in
+    if
+      builtins.attrNames authoredArgs == [ "system" ]
+      && environment != null
+      && environment.system == system
+      && (
+        environment.nixpkgsSource == toString nixpkgs
+        || ((nixpkgs.narHash or null) != null && nixpkgs.narHash == (environment.narHash or null))
+      )
+    then
+      environment.pkgs
+    else
+      import nixpkgs importArgs;
   mkCaminoReactContracts = import ./react-contracts.nix;
   mkCaminoYarnDependencies = import ./yarn-dependencies.nix;
   mkCaminoYarnShell = import ./yarn-shell.nix;