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.

README.md

@@ -18,6 +18,9 @@ through configurable chains of SOCKS4, SOCKS5, and HTTP CONNECT proxies.
 - Dead proxy reporting to upstream API (optional `report_url`)
 - SIGHUP hot reload (timeout, retries, log_level, pool config)
 - Connection metrics with pool stats (logged periodically and on shutdown)
+- Concurrent connection limit with backpressure (`max_connections`)
+- Async HTTP client for proxy source fetching (parallel, no threads)
+- First-hop TCP connection pool (pre-warmed, stale-evicted)
 - Container-ready (Alpine-based, podman/docker)
 - Graceful shutdown (SIGTERM/SIGINT)
 - Pure Python, asyncio-based, minimal dependencies
@@ -65,6 +68,8 @@ cp config/example.yaml config/s5p.yaml
 listen: 127.0.0.1:1080
 timeout: 10
 retries: 3
+max_connections: 256                  # concurrent connection limit
+pool_size: 8                          # pre-warmed connections to first hop
 
 chain:
   - socks5://127.0.0.1:9050          # Tor
@@ -84,7 +89,7 @@ proxy_pool:
 ## CLI Reference
 
 ```
-s5p [-c FILE] [-l [HOST:]PORT] [-C URL[,URL,...]] [-S URL] [-t SEC] [-r N] [-v|-q]
+s5p [-c FILE] [-l [HOST:]PORT] [-C URL[,URL,...]] [-S URL] [-t SEC] [-r N] [-m N] [-v|-q]
 
 Options:
   -c, --config FILE        YAML config file
@@ -93,6 +98,7 @@ Options:
   -S, --proxy-source URL   Proxy source API URL
   -t, --timeout SEC        Per-hop timeout (default: 10)
   -r, --retries N          Max attempts per connection (default: 3, proxy_source only)
+  -m, --max-connections N  Max concurrent connections (default: 256)
   -v, --verbose            Debug logging
   -q, --quiet              Errors only
   --cprofile [FILE]        Enable cProfile, dump to FILE (default: s5p.prof)