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.

build-profile.mjs

@@ -0,0 +1,12 @@
+/** Timing is log data, never part of an artifact or its witness. */
+export function measureBuild(phase, details, action) {
+  const started = performance.now();
+  try {
+    return action();
+  } finally {
+    if (process.env.QUIXOS_TRACE_PROFILE === "1")
+      console.error(
+        `[qx-build-profile] ${JSON.stringify({ phase, ms: Math.round(performance.now() - started), ...details })}`,
+      );
+  }
+}