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_group.c

@@ -11,6 +11,8 @@
 
 static int group_fired = 0;
 static int other_fired = 0;
+static int group_status = 0;
+static int other_status = 0;
 
 akerr_ErrorContext *boom(int status)
 {
@@ -28,8 +30,10 @@ akerr_ErrorContext *run(int status)
     } HANDLE(e, AKERR_KEY)
 	  HANDLE_GROUP(e, AKERR_INDEX) {
 	group_fired++;
+	group_status = e->status;
     } HANDLE(e, AKERR_IO) {
 	other_fired++;
+	other_status = e->status;
     } FINISH(e, false);
     return e;
 }
@@ -40,15 +44,18 @@ int main(void)
 
     run(AKERR_KEY);
     AKERR_CHECK(group_fired == 1);
+    AKERR_CHECK(group_status == AKERR_KEY);
     AKERR_CHECK(other_fired == 0);
 
     run(AKERR_INDEX);
     AKERR_CHECK(group_fired == 2);
+    AKERR_CHECK(group_status == AKERR_INDEX);
     AKERR_CHECK(other_fired == 0);
 
     run(AKERR_IO);
     AKERR_CHECK(group_fired == 2);
     AKERR_CHECK(other_fired == 1);
+    AKERR_CHECK(other_status == AKERR_IO);
 
     AKERR_CHECK(akerr_slots_in_use() == 0);
     fprintf(stderr, "err_handle_group ok\n");