feat(canvas): open .canvas files in a workspace pane (T-322)

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>

CHANGELOG.md

@@ -18,6 +18,10 @@ heading, and (b) bumping `pubspec.yaml` `version:` in the same commit.
 
 ### Added
 
+- **Canvas pane.** `.canvas` (Obsidian JSONCanvas) files now open in an
+  interactive workspace pane — each document a sub-tab, with pan, zoom, and
+  node selection. Routed from the file tree, quick-open, and
+  `clide ui open canvas <path>`. (T-322)
 - **Vault graph view.** A force-directed link graph of the whole vault in the
   context panel — notes are nodes, wikilinks edges. Hover highlights a note's
   neighbourhood; click opens it. Scroll to zoom, drag to pan; filter by path