fix(ergasia,exousia): ref-count shared torrent_ids and constant-time login miss-path (#536, #540)

[forkwright]

Two verified fixes from the 2026-07-03 deep-audit workflow (adversarially verified + Opus-judged).

#536 — deleting one download destroyed a sibling's live torrent (ergasia). librqbit dedups a duplicate info-hash onto one torrent_id (returns AlreadyManaged), so two DownloadIds could map to the same torrent_id; delete_torrent then unconditionally called session.delete() on that shared id, destroying the other download's still-live torrent until a restart reconcile pruned it. Added a torrent_refcounts: DashMap<usize, usize> reverse index: acquire_torrent_ref on every mapping insert (add, delete-failure restore, reconcile-from-disk), release_torrent_ref on delete. session.delete() now runs only when the ref count hits zero; an earlier owner just drops its own mapping entry and leaves the torrent live. The decrement-and-test runs entirely inside DashMap's Entry shard lock, so two concurrent releases for one torrent_id can never both observe "last reference". Regression test (multi_thread) registers two DownloadIds onto one torrent_id, deletes the first, asserts the second's stats still resolve and the librqbit torrent is still present, then deletes the second and asserts it's gone.

#540 — login timing side-channel enabled username enumeration (exousia, SECURITY / CWE-203). login() returned InvalidCredentials immediately on a username miss but ran a full Argon2id verify when the username existed, so response latency alone distinguished existing from non-existing accounts. Fixed by routing both branches through a single verify_password call: against the real hash when the row exists, otherwise against a fixed precomputed Argon2id sentinel hash (default m=19456,t=2,p=1 params matching hash_password), so a non-existent (or too-corrupt-to-convert) username costs the same Argon2id work and wall-clock time as a real wrong-password attempt. The verify runs before the is_active check, keeping inactive accounts indistinguishable too. A #[cfg(test)] thread-local verify-call spy proves both miss-paths exercise exactly one Argon2id verify; a third test guards that the sentinel constant parses as valid PHC and never matches.

Gate: kanon gate --full green — fmt, check, clippy (-D warnings), nextest 1919 passed, deny, kanon lint (0/0/0). Two gate iterations fixed a collapsible_if, a deprecated-term lint, and a no-silent-result-swallow at root (not suppressed). Gate-Passed trailer stamped.

Closes #536
Closes #540