[PATCH 3/3] lib-storage: thread - Limit ancestor chain traversal depth to prevent...

[PATCH 3/3] lib-storage: thread - Limit ancestor chain traversal depth to prevent O(N^2) CPU usage

The per-message References limit (MAIL_THREAD_REFERENCES_MAX) prevents a
single crafted message from causing O(N^2) traversals in
thread_node_has_ancestor(). However, multiple crafted messages each
containing 1000 References entries can build an arbitrarily deep ancestor
chain across the mailbox, causing the same quadratic blowup spread over
many messages: processing email k costs O(k * MAIL_THREAD_REFERENCES_MAX)
steps, giving O(M^2 * MAIL_THREAD_REFERENCES_MAX) total for M emails.

Fix this by limiting the traversal depth in thread_node_has_ancestor() to
MAIL_THREAD_REFERENCES_MAX steps. When the limit is reached the link is
dropped, bounding per-email work to O(MAIL_THREAD_REFERENCES_MAX^2)
regardless of how deep the chain was built by prior messages.

Gbp-Pq: Name 0003-lib-storage-thread-Limit-ancestor-chain-traversal-de.patch