body-drain: add -c concurrency and -idle-per-host to the client

Bounded concurrency via semaphore+WaitGroup, atomic counters, stop-on-first-
error without cancelling in-flight requests (cancel would hide the leaks).
Exposes MaxIdleConnsPerHost so pool thrash under concurrency is observable.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GbnJj9ZqEx9nUvzkjdwfMq

99-experiments/body-drain/README.md

@@ -8,6 +8,33 @@ go run ./99-experiments/body-drain -mode all
 go run ./99-experiments/body-drain -mode leak -n 200
 ```
 
+## Concurrency: `-c` and `-idle-per-host`
+
+`-c N` keeps up to N requests in flight (semaphore + WaitGroup, the same
+shape as exercise 01). `-idle-per-host` sets `Transport.MaxIdleConnsPerHost`
+(default 10; Go's own default is only **2**).
+
+```
+go run ./99-experiments/body-drain -mode drain -n 200 -c 8
+#   httptrace GotConn  : reused=192 fresh=8      ← one conn per worker, then reused
+#   server new conns   : 8
+
+go run ./99-experiments/body-drain -mode drain -n 200 -c 8 -idle-per-host 2
+#   server new conns   : 13+                     ← pool too small: conns returned
+#                                                   while 2 are already idle get CLOSED
+```
+
+That second run is the trap in exercise 01: `limit` goroutines against one
+host with the default transport means only 2 conns are ever kept, the rest are
+dialed and thrown away on every round — even though you drained the body
+correctly. On loopback the effect is small; against a TLS host each throwaway
+is a full handshake. Raise `MaxIdleConnsPerHost` to your concurrency.
+
+With `-c` the leak modes also exhaust fds N times faster, and failures no
+longer stop in-flight requests: cancelling a request context closes its body
+and would clean up the very leaks we want to see, so the client only stops
+*launching* new requests after the first error.
+
 ## Two processes: watch each side run out of fds on its own
 
 In one process the client and server share one fd limit and each leaked