A fresh from-scratch canvas holds only the blank starter frame, and that
frame ships selected. A whole-new-design request phrased as a bare noun
phrase (no page/screen noun, no create verb — e.g. "Luxury webapp for
managing barbershop clients") slips past both new-screen exemption gates,
and the selected starter makes selected_target_instruction true, so the
request was launched as a modify turn on a near-empty canvas — the model
then emits an unparseable fragment and the user sees "Could not parse
design nodes". Guard should_launch_direct_modify: a canvas that is only
the blank starter frame has nothing real to modify, so any request on it
is NEW (desktop parity of web_chat_standard's page_children_empty => New).
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