httputil: ResponseChain can exhaust or deadlock the buffer pool #799

ResponseChain can block forever on the httputil buffer pool:

• NewResponseChain takes its two buffers one at a time. With N concurrent chains on an N-slot pool, each holds one slot and waits for the second.
• FullResponse() takes a third pooled buffer that external callers cannot return (putBuffer is unexported), so every call leaks a slot until the pool is exhausted. Used by proxify and nuclei-internal-dast.

Surfaced in #793 (TestResponseChain_BurstWithPoolExhaustion hung once pool shrinking actually worked).