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_handle_default.c

@@ -8,6 +8,7 @@
 
 static int specific_fired = 0;
 static int default_fired = 0;
+static int default_status = 0;
 
 akerr_ErrorContext *boom(void)
 {
@@ -28,10 +29,12 @@ int main(void)
 	specific_fired = 1; /* must NOT run: error is AKERR_TYPE */
     } HANDLE_DEFAULT(e) {
 	default_fired = 1;
+	default_status = e->status;
     } FINISH_NORETURN(e);
 
     AKERR_CHECK(specific_fired == 0);
     AKERR_CHECK(default_fired == 1);
+    AKERR_CHECK(default_status == AKERR_TYPE);
     AKERR_CHECK(akerr_slots_in_use() == 0);
     fprintf(stderr, "err_handle_default ok\n");
     return 0;