Preserve validation cancellation during reader shutdown

docs/internal/validation.md

@@ -54,7 +54,8 @@ neighboring files. Each tree owns one lazy Git blob reader; direct tree users
 must close it when finished. The shared validation executor closes its tree on
 every return before product checks or build callbacks run. A metadata-only
 selection never starts the reader. Per-read contexts cancel an in-flight blob
-exchange without tying the reader lifetime to a short-lived caller context.
+exchange. The context supplied when creating the tree governs the reader
+lifetime, including shutdown and child reaping.
 Regular-file reads return independent caller-owned byte slices and are not
 retained by the tree. Symlink targets are retained as immutable owned strings
 by blob object ID; explicitly reading a symlink still returns independent