Test the installed package, not just the build tree

Found by building a consumer against a fresh install: an installed
libakstdlib was not usable. akstdlibConfig.cmake calls
find_dependency(akerror), and the top-level build pulls the submodule in
EXCLUDE_FROM_ALL, so `cmake --install` on this project installs only
this project and leaves that dependency unresolvable.

CI had been hiding it by installing libakerror@main in a separate step
-- a different libakerror from the one actually compiled, which is the
inconsistency TODO.md 2.3 recorded and the previous commit removed.
Removing it exposed the real gap.

CI now installs deps/libakerror, the same commit the top-level build
compiles, so there is one libakerror in play and the install is
consumable.

tests/consumer/ is the check that would have caught it: a standalone
project, configured against CMAKE_PREFIX_PATH rather than as part of
this build, because being part of this build is exactly what would let
it pass without testing anything. It exercises what only an install has
-- find_package with a version request against the generated version
file, find_dependency(akerror) resolving, and the exported
akstdlib::akstdlib target -- and touches one function from each of the
four sources, so a library installed with a source file missing from its
link line fails there rather than in the next consumer to find it.

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

AGENTS.md

@@ -19,7 +19,9 @@ reasoning. The implementation is split by domain:
 
 Tests are one-file CTest executables under `tests/test_<name>.c`, with shared
 helpers in `tests/aksl_capture.h`. `tests/negative/` holds sources that must
-*fail* to compile -- see the testing section below. CMake package templates are
+*fail* to compile, and `tests/consumer/` is a standalone project built against
+the *installed* package rather than the build tree -- see the testing section
+below. CMake package templates are
 in `cmake/` and `akstdlib.pc.in`. The vendored dependency is `deps/libakerror`;
 update it as a submodule rather than editing generated files under `build/`,
 `build-asan/` or `build-coverage/`.
@@ -108,6 +110,12 @@ guarantees the compiler enforces and nothing else does: `AKERR_NOIGNORE` and the
 format attributes. Drop either in a refactor and every ordinary test still
 passes.
 
+`tests/consumer/` is configured standalone against `CMAKE_PREFIX_PATH`, not as
+part of this build, because being part of this build is what would let it pass
+without testing anything. It covers the paths only an install has: the version
+file, `find_dependency(akerror)` resolving, and the exported
+`akstdlib::akstdlib` target. CI runs it after `cmake --install`.
+
 Coverage is 99.5% of lines and 100% of functions across all four sources; CI
 gates at 90 (line) / 40 (branch), so new code needs tests in the same commit. Run
 `cmake --build build-coverage --target coverage` and check the uncovered-line