Fills a real gap in the element-tool family: upload / drag-and-
drop surfaces. Dashed border + cloud icon + two-line instruction
("Drop files to upload" / "or click to browse") — the classic
pattern from every modern file-upload UI.
Deliberately structurally similar to add_empty_chart_v0 (dashed
border + icon + title/subtitle) but semantically distinct:
- empty_chart = "chart widget will render when data arrives"
(320×200, icon chart-typed)
- upload_dropzone = "users drop files here" (480×200, icon
semantic: upload-cloud / upload / file-up)
elements.md routes them by intent, and the tool descriptions
cross-reference each other to prevent the AI from picking the
wrong one on ambiguous prompts.
Wired through all the standard points per the add-new-tool
checklist: builder + handler + schema (into ext-1, the shorter
shard) + shim + SERVER_BUILDERS + parity CASES + contract
allow-list + elements.md decision tree + keyword triggers +
minimal usage. Handler test covers 5 cases: defaults, dashed
stroke, overrides, size clamping, bogus parent_id rejection.
45214efb70