docs: document connection limit, async fetch, and connection pool

Update USAGE, CHEATSHEET, README, PROJECT, and TASKS for the three
performance improvements: max_connections semaphore, async HTTP
source fetching, and first-hop TCP connection pool.

TASKS.md

@@ -27,6 +27,9 @@
 - [x] Static chain health check (skip pool tests if chain unreachable)
 - [x] SIGHUP hot config reload (timeout, retries, log_level, pool config)
 - [x] Dead proxy reporting (`report_url` POST evicted proxies to API)
+- [x] Concurrent connection semaphore (`max_connections`, CLI `-m`)
+- [x] Async HTTP client (replace blocking urllib, parallel source fetch)
+- [x] First-hop TCP connection pool (`pool_size`, `pool_max_idle`)
 
 ## Next
 - [ ] Integration tests with mock proxy server