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.

docs/CHEATSHEET.md

@@ -13,6 +13,7 @@ s5p -v                                      # debug logging
 s5p -q                                      # errors only
 s5p -S http://api:8081/proxies              # proxy source API
 s5p -r 5                                   # retry up to 5 proxies
+s5p -m 512                                 # max concurrent connections
 s5p --cprofile                              # profile to s5p.prof
 s5p --cprofile out.prof                     # profile to custom file
 ```
@@ -34,6 +35,14 @@ Volumes: `./src` (ro), `./config/s5p.yaml` (ro), `~/.cache/s5p` → `/data` (poo
 cp config/example.yaml config/s5p.yaml     # create live config (gitignored)
 ```
 
+## Performance Tuning (config)
+
+```yaml
+max_connections: 256   # concurrent connection cap
+pool_size: 8           # pre-warmed TCP conns to first hop (0 = off)
+pool_max_idle: 30      # evict idle pooled conns (seconds)
+```
+
 ## Proxy Pool (config)
 
 ```yaml