Expand error status test coverage

- Added explicit status validation across error handling tests.
- Added lifecycle and slot-leak checks to older tests.
- Improved unhandled-error propagation coverage.
- Added repository guidance and a test runner script.
- Verified all 23 CTest tests pass.

Co-Authored by Codex GPT 5.4

tests/err_pass.c

@@ -35,6 +35,7 @@ int main(void)
     } CLEANUP {
     } PROCESS(e) {
     } HANDLE(e, AKERR_IO) {
+	AKERR_CHECK_STATUS(e, AKERR_IO);
 	handled = 1;
     } FINISH_NORETURN(e);