An archetype's meta gets an optional icon, the SVG text (at most 16 KiB), which
the controller checks, stores and serves on GET /api/archetypes. The built-in
archetypes read theirs from nix/archetypes/icons. The create form shows each
archetype as a tile with its icon and name, laid out like the broadcast screen's
agent tiles, in place of the dropdown, and draws the SVG through an img data URL.
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