BUILD-CORNER-WIDGET-CLASS-RUDDER-IN-PLUGIN-STUDIO (design approved by dx-shepherd). A profile's .meta.json
declares "widget": { "corner": "bottom-right" | ... }; generate-tree carries it into manifest.json, so the
Studio keeps no list. The Studio mounts each widget once in a fixed corner host (z-index above everything),
outside the rail and the pane, collapsed to a button, with its open state remembered. Picking it in the rail
opens the corner instead of mounting a second copy in a pane. The host and the mounted element live in a
page-level singleton on document.body, so a Studio hot update or remount re-attaches the same element and
Rudder keeps its live stream. rudder-voice declares bottom-right. Needs eyes: open, HMR, collapse.
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