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>

assets/i18n/en_us/builtin.canvas.json

@@ -0,0 +1,5 @@
+{
+  "tab.title": { "translation": "Canvas" },
+  "empty": { "translation": "Open a .canvas file to view it here." },
+  "status.loading": { "translation": "Loading…" }
+}

assets/i18n/nl_nl/builtin.canvas.json

@@ -0,0 +1,5 @@
+{
+  "tab.title": { "translation": "Canvas" },
+  "empty": { "translation": "Open een .canvas-bestand om het hier te bekijken." },
+  "status.loading": { "translation": "Laden…" }
+}