CHW-1306 DeepScanNotSequenced

Default message: run the basic scan for this coordinate before requesting a deep scan
HTTP status: 409 (client)
Reason: NOT_SEQUENCED

Problem. A public deep scan was requested for a coordinate that has no report yet. Cause. The deep scan fetches a real artifact from an upstream registry — the only public path that originates egress on a caller’s say-so — so it requires the basic scan to have run first. Sequencing is a control, not a convenience: it makes the per-IP limiter and the Turnstile gate on the basic /scan endpoint an unavoidable prerequisite, rather than a door the deep endpoint could be used to walk around. Fix. Request the basic scan for the coordinate first, wait for it to produce a report, then request the deep scan. HTTP 409.