Preserve ignored error snapshots

docs/thread-safety.md

@@ -14,11 +14,12 @@ What that covers:
   against each other and against lookups. Two threads reserving the same range
   cannot both win — exactly one gets `NULL` and the other gets
   `AKERR_STATUS_RANGE_OVERLAP` naming the winner.
-* **Per-thread state.** `IGNORE` uses `__akerr_last_ignored` as a scratch pointer
-  while it logs an error, then releases the context and clears the pointer.
-  That scratch pointer and the last-ditch context used to report
-  `akerr_release_error(NULL)` are thread-local, so concurrent calls cannot
-  overwrite each other's state.
+* **Per-thread state.** `IGNORE` copies the swallowed context into its
+  thread-local `__akerr_last_ignored` snapshot before releasing the pool slot.
+  The snapshot remains valid until that thread ignores another error, so a
+  later pool checkout cannot overwrite it. The snapshot and the last-ditch
+  context used to report `akerr_release_error(NULL)` are thread-local, so
+  concurrent calls cannot overwrite each other's state.
 * **Handing a context from one thread to another.** A context is not thread
   state — it lives in `AKERR_ARRAY_ERROR`, which is process-global — so it
   outlives the thread that raised it. The reference count is the only field the