A detached session no client watches draws no frames: yes | head -c 50M through pty/run goes from over 30 s to 11 s

Profiling the 50 MB run showed 70% of its time in draw.render,
recolorAnsi alone 43%, walking cells through PageList.pin. The detached
session drew a frame after every pty read, with no client attached to
see it. A host now says whether anyone watches (Host.VTable.watched). The
detached server answers whether a client is attached, and while none is
the core keeps the frame owed instead of drawing it. A /screen read draws
its own, and a client attaching gets one (hello sets needs_frame).

Debug, 80x24: 10 MB went from 7.36 s to 2.10 s, and 50 MB from over the
30 s client timeout (about 37 s by rate) to 11.08 s. What is left is the
terminal's own parsing and scrollback growth. A session a frontend shows
still draws after every read. Pacing those frames is the next step if
that case matters.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

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