Mirrors image.show: a Flutter-free `draw` handler reads + JSON-parses the
document (injected reader), lowers it to SVG via the template registry,
and publishes {svg, label, description} on the `draw` MessageBus channel
for the Claude extension to inject. Honest errors — notFound (missing
file), userError (bad JSON / unknown template / no --file), toolError (no
live UI). Covered by dart test (7 cases). The extension subscription that
injects the card remains to wire.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
jpmschweitzerandClaude Opus 4.8
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