Guard the add_test shadow on being top-level again

CMake chains command overrides one level deep: a second override rebinds
_add_test to the first and the builtin becomes unreachable to everyone. So two
projects in one tree cannot both shadow add_test, and akbasic has to shadow it
first for libakerror and libakstdlib -- which left its own registrations
recursing to CMake's depth limit.

The unconditional shadow arrived with the vendored-dependency fix in 18399f2.
That half stays; only the guard comes back.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

TODO.md

@@ -1278,6 +1278,41 @@ Every size below is measured, not estimated.
   `amdgpu_dri.so`; running it headless produces none, and headless is what the
   suites are written for anyway.
 
+### Found while embedding libakgl in a consumer
+
+40. ~~**Shadowing `add_test()` unconditionally broke every embedding consumer.**~~ **Fixed in
+    the same release it was introduced in**, and worth keeping because the CMake behaviour
+    behind it is not obvious and will catch the next person.
+
+    18399f2 moved the vendored-dependency block out from behind
+    `if(CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR)` — which was the point of that
+    commit — and took the `add_test()`/`set_tests_properties()` override out with it. The
+    override's own comment claimed "an embedding consumer's `add_test()` still reaches CTest".
+    It does not, and cannot.
+
+    **CMake chains command overrides exactly one level deep.** Overriding `add_test` makes the
+    builtin available as `_add_test`; overriding it a *second* time rebinds `_add_test` to the
+    *first* override and the builtin becomes unreachable to everybody, permanently — there is
+    no `__add_test`. Verified directly rather than inferred from the documentation:
+
+    ```cmake
+    function(add_test) _add_test(${ARGV}) endfunction()   # _add_test  -> builtin
+    function(add_test) _add_test(${ARGV}) endfunction()   # _add_test  -> the first override
+    if(COMMAND __add_test) ... endif()                    # absent
+    ```
+
+    So two projects in one tree cannot both shadow it. `akbasic` shadows it first — it has to,
+    for `libakerror` and `libakstdlib`, which register their tests unconditionally — and its own
+    registrations then recursed until CMake stopped at "Maximum recursion depth of 1000
+    exceeded". A consumer had no way to work around it: once the builtin is gone it is gone for
+    that project too.
+
+    **Fix:** the override is guarded on being top-level again, exactly as it was before 18399f2
+    and exactly as `libakstdlib` guards its own. The vendored-dependency block stays
+    unconditional, which is what that commit was actually for. An embedded `libakgl` leaves
+    `add_test` alone and lets its consumer suppress what it does not want — machinery the
+    consumer has to have anyway.
+
 ## Build notes
 
 The vendored SDL satellite libraries are built into per-project subdirectories