Just: say why these two imports are in this order

The first definition of a variable wins, so the optional internal file has
to come before the stub that stands in for it. Reversed, the stub wins even
where the real file exists and that checkout quietly behaves as if it were
external.

Worth a comment rather than trusting the reader's intuition, which says
last wins, and because this repository cannot catch the mistake: the file
is never present here, so both orders evaluate identically.

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Author: redsun82

misc/just/defs.just

@@ -1,3 +1,8 @@
+# The first definition of a variable wins, not the last, so this order is what lets the
+# internal file set `SEMMLE_CODE` while the stub is the fallback when it is absent.
+# Swapped, the stub wins even where the real file exists and that checkout then behaves
+# as if it were external. Nothing here can catch that: with the file absent, as it
+# always is in this repository, both orders evaluate the same.
 import? '../../../semmle-code.just' # internal repo just file, if present
 import 'semmle-code-stub.just'