Derive err_maxval's code set from the header, not a hardcoded list

The previous err_maxval hardcoded the list of AKERR_* codes, which silently
rots the moment a code is added. Instead, parse the generated akerror.h at
runtime: discover every "#define AKERR_<NAME> (AKERR_LAST_ERRNO_VALUE + N)",
take the highest offset actually defined, and assert AKERR_MAX_ERR_VALUE covers
it. A compile-time cross-check ties the parsed ceiling to the compiled macro so
the test can't pass by reading a stale header.

CMake injects the header path as AKERR_GENERATED_HEADER. Verified: the test
fails (max_err_value >= highest_code) when pointed at a +15 header while
AKERR_BADEXC is +17, and now also strengthens header mutation coverage.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Co-Authored-By: Andrew Kesterson <andrew@aklabs.net>

CMakeLists.txt

@@ -81,6 +81,11 @@ foreach(_test IN LISTS AKERR_TESTS)
     add_test(NAME ${_test} COMMAND test_${_test})
 endforeach()
 
+# err_maxval parses the generated header to check AKERR_MAX_ERR_VALUE covers
+# every AKERR_* code, so it needs the path to the header it was built against.
+target_compile_definitions(test_err_maxval PRIVATE
+    AKERR_GENERATED_HEADER="${GENERATED_AKERROR_H}")
+
 set_tests_properties(
     ${AKERR_WILL_FAIL_TESTS}
     PROPERTIES WILL_FAIL TRUE