feat(proxy): add offset param to inset or outset traced shapes

Trace results can now be inset (negative) or outset (positive) by a
pixel amount. Offset the outline and holes in opposite directions with
clipper-lib's miter joins, then recombine with a boolean difference so
holes grow on inset and shrink on outset. Collapsed shapes return an
empty outline; a split outline keeps the largest ring. Document the
parameter and the new dependency.

apps/proxy/package.json

@@ -18,12 +18,14 @@
     "@tts/tts": "workspace:*",
     "@visioncortex/vtracer": "1.0.0-alpha.3",
     "bson": "^6.10.4",
+    "clipper-lib": "^6.4.2",
     "hono": "^4.6.14",
     "sharp": "^0.35.3",
     "svgpath": "^2.6.0",
     "zod": "^3.24.1"
   },
   "devDependencies": {
+    "@types/clipper-lib": "^6.4.0",
     "@types/node": "^22.10.2",
     "tsx": "^4.19.2",
     "typescript": "^5.7.2"