gateway_attach_bench.py drives the real dispatcher with a session whose agent
build is still running and times each attach RPC against prompt.submit as the
control — the harness that located the stall and measures it.
image-attach-bench.mjs times the renderer-side transforms (file read, base64,
RPC frame, embedded-image extraction, render-weight walk) across image sizes.
It is what ruled the renderer out: ~26ms total at 3MB.
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