Add IWS=0 / slow-read DoS PoC test tool.

Self-contained test tool that spins up an in-process origin server
and an ATS runroot, then runs either an HTTP/2 INITIAL_WINDOW_SIZE=0
attack or an HTTP/1 slow-read attack, measuring per-stream RSS delta
against the 1 MB limit.  Requires a malloc-linked ATS build so that
freed IOBuffers return memory to the OS rather than the freelist.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: bryancall