feat(canvas): add CAD vector semantic context and non-destructive vector crop

Extend CAD conversion to emit bounded semantic metadata (units, layers,
blocks, entity types, labels, crop-indexed bounds) alongside the derived
SVG, tag imported drawings as cad-vector nodes, and carry semanticContext
through canvas nodes, snapshot normalization, and reference images. Agent
Chat forwards this metadata as hidden cad-context reference parts.

Add non-destructive vector cropping: cropping a vector/CAD node keeps the
source SVG and spawns a transparent WebP sibling with crop-scoped semantic
context, wired through the vectorized image toolbar and the workspace crop
tool. Include node --test coverage for CAD semantics and vector crop.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/ui/pages/CanvasWorkspace/canvas/nodeReferenceImages.ts

@@ -12,7 +12,8 @@ export function canvasNodeToReferenceImage(node: CanvasNode): UploadedReferenceI
     id: `${canvasNodeReferencePrefix}${node.id}`,
     name: node.title.trim() || "Image",
     publicUrl: content,
-    previewUrl: content
+    previewUrl: content,
+    semanticContext: node.semanticContext
   };
 }