POST /tool/signup upserts a Viewer keyed on the resolved request UDID
(via the existing SID->UDID dict). Stashes the viewer on HttpContext so
the translation middleware emits viewer_id/short_udid/udid in
data_headers. Empty data payload -- all signup outputs flow in
data_headers per spec. Idempotent: repeat signups for the same UDID
return the existing viewer.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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