docs: update project docs for backoff, stale expiry, pool metrics

Add failure backoff and stale expiry sections to USAGE. Document pool=
field in metrics output. Update ROADMAP, TASKS, TODO with completed
items and remaining suggestions. Add metrics example to CHEATSHEET.

TODO.md

@@ -7,6 +7,9 @@
 - Per-destination chain rules (bypass chain for local addresses)
 - Hot config reload on SIGHUP
 - Systemd socket activation
+- Parallel health tests at startup (fast warm start)
+- Report dead proxies back to source API
+- Chain health check (test static chain before pool tests)
 
 ## Performance