The gap test proves codec correctness across a drop; this one runs the real
server loop and proves the drop mechanism fires: a client reporting a high
decode backlog gets a stream with skipped frame indices (maxGap ~= MAX_CONSEC_DROPS),
while a client reporting zero backlog gets every frame in order. Spawns
stream_server.py against an ffmpeg-generated clip; keeps stdin open because the
server's command loop runs on the main thread and dies on EOF.
