Makes the canvas foundation (parser/renderer/view) reachable. The
extension owns an app-scoped MultitabController (the diff/T-233
pattern) so open documents survive the pane being rebuilt; each
document is a real sub-tab per the refinement decision, kept alive
across switches. Routing goes through the existing seams instead of
the dead TabContribution.fileGlobs field: openWorkspaceFile gains a
.canvas branch mirroring .md, and ui.open gains a canvas reader for
D-6 parity (clide ui open canvas <path>). Also corrects the
json_canvas doc header that claimed SVG-lowering — the interactive
pane paints the model directly (D-103 live-widget exception).
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
jpmschweitzerandClaude Fable 5
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