test: document module name shadowing between stanza and library

When an internal module name shadows an unwrapped library's module name,
the internal module takes precedence and the library's module is
inaccessible. This means ocamldep will report the internal module, not
the library's, so dependency filtering that treats stanza-internal names
as non-library references is correct.

Signed-off-by: Robin Bate Boerop <me@robinbb.com>

Author: robinbb

test/blackbox-tests/test-cases/per-module-lib-deps/module-name-shadowing.t

@@ -0,0 +1,62 @@
+Test that when a stanza's internal module name shadows a library module name,
+the internal module takes precedence. This validates that ocamldep-based
+dependency filtering (which treats names found in stanza_modules as internal)
+correctly reflects the compiler's resolution order.
+
+See: https://github.com/ocaml/dune/issues/4572
+
+  $ cat > dune-project <<EOF
+  > (lang dune 3.0)
+  > EOF
+
+--- Unwrapped library: internal module shadows library module ---
+
+An unwrapped library exposes module Helper. The executable also has a module
+named Helper. The internal module takes precedence — the library's Helper is
+inaccessible.
+
+  $ mkdir unwrapped_lib
+  $ cat > unwrapped_lib/dune <<EOF
+  > (library
+  >  (name unwrapped_lib)
+  >  (wrapped false))
+  > EOF
+
+  $ cat > unwrapped_lib/helper.ml <<EOF
+  > let lib_value = 42
+  > EOF
+
+  $ cat > unwrapped_lib/helper.mli <<EOF
+  > val lib_value : int
+  > EOF
+
+  $ cat > dune <<EOF
+  > (executable
+  >  (name main)
+  >  (libraries unwrapped_lib))
+  > EOF
+
+  $ cat > helper.ml <<EOF
+  > let local_value = 1
+  > EOF
+
+  $ cat > main.ml <<EOF
+  > let () = print_int Helper.local_value
+  > EOF
+
+The build succeeds using the internal Helper:
+
+  $ dune build ./main.exe
+
+The library's Helper.lib_value is not accessible:
+
+  $ cat > main.ml <<EOF
+  > let () = print_int Helper.lib_value
+  > EOF
+
+  $ dune build ./main.exe 2>&1
+  File "main.ml", line 1, characters 19-35:
+  1 | let () = print_int Helper.lib_value
+                         ^^^^^^^^^^^^^^^^
+  Error: Unbound value Helper.lib_value
+  [1]