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/i18n/locales/en-US.json

@@ -583,6 +583,8 @@
   "adjustImage": "Adjust",
   "adjustPrompt": "Apply the adjustment parameters to improve light, color, contrast, and detail; if no values are set, automatically improve the overall look while preserving subject and composition.",
   "cropImage": "Crop",
+  "cropResultTitle": "{title} crop",
+  "cropCreated": "Created a cropped image beside the source",
   "cropPrompt": "Recompose the image using the selected crop region and keep the important frame.",
   "cropPresets": "Presets",
   "cropPresetGeneral": "General",