Add PF synlimit backend

Implement synlimit = "pf" for PF-based firewalls using per-listener source tracking rules in a Telemt-owned anchor.

PF evaluates max-src-conn-rate through source tracking when creating
tates for completed TCP connections. Telemt installs a rate-limited pass
rule followed by a reject fallback rule in the listener anchor:

  - under-limit new connections create PF state and pass;
  - over-limit new connections fall through to block return-rst;
  - access resumes automatically when the source rate drops below the configured window.

Keep Linux iptables/nftables behavior unchanged.
PF maps synlimit_hitcount / synlimit_seconds to max-src-conn-rate and intentionally has
no direct equivalent for Linux-only burst/hashlimit knobs.

src/synlimit_control/iptables.rs

@@ -321,6 +321,7 @@ mod tests {
             nft_table: "telemt_synlimit_test".to_string(),
             iptables_chain: "TMT_SYN_TEST".to_string(),
             iptables_hashlimit_prefix: "TMTTEST".to_string(),
+            pf_anchor: "telemt_synlimit/test".to_string(),
         }
     }