docs: update TODO.md with pool-related backlog items

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

TODO.md

@@ -6,6 +6,9 @@
 - Chain randomization modes (round-robin, sticky-per-destination)
 - Per-destination chain rules (bypass chain for local addresses)
 - Systemd socket activation
+- Per-pool health test chain override (different base chain per pool)
+- Pool-level proxy protocol filter (only socks5 from pool X, only http from pool Y)
+- Listener-level retry override (different retry count per listener)
 
 ## Performance
 
@@ -19,6 +22,12 @@
 - Rate limiting per source IP
 - Access control lists
 
+## Observability
+
+- Prometheus metrics endpoint (`/metrics` in OpenMetrics format)
+- Per-pool health test success rate tracking
+- Per-pool latency breakdown in `/status`
+
 ## Docs
 
 - Man page