chore(quest): plan the stock fixture's target dir race

quest/m0/checks/README.md

@@ -23,3 +23,4 @@ Each child reports its CPU saved, cold and warm, against `origin/main`.
 - [A warm gate](/quest/m0/checks/cache.md) - the land gate builds through mbx, as moq's does
 - [Stable names on a small fixture](/quest/m0/checks/stable-names.md) - `stable_names` runs on a small fixture with generic instances and trait impls
 - [Fixtures that merge](/quest/m0/checks/fixture-layout.md) - `lang` splits by module, and `reject`'s snapshots key by item, not line
+- [The stock fixture's target dir race](/quest/m0/checks/stock-cachedir.md) - the `stock` fixture never fails creating its target dir

quest/m0/checks/stock-cachedir.md

@@ -0,0 +1,15 @@
+# [S] The stock fixture's target dir race
+
+## Goal
+
+The ts `stock` fixture never fails setting up its build, as it did once in the land fixer
+with `writing .../CACHEDIR.TAG: No such file or directory`, and passed on a rerun.
+
+## Plan
+
+Found by the land fixer on 2026-11-02, running #391. Cargo writes `CACHEDIR.TAG` when it
+creates a target dir, so the likely cause is two tests, or a test and a recipe, sharing one
+target dir while another removes or recreates it. Reproduce it under parallel runs, and fix
+the race at its source, never by retrying.
+
+Public API: none.