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.

packages/shared/src/schemas.ts

@@ -65,4 +65,6 @@ export const traceRequestSchema = z.object({
   format: traceFormatSchema.default('shape'),
   simplify: z.coerce.number().min(0).optional(),
   maxColors: z.coerce.number().int().min(1).optional(),
+  /** Inset (negative) or outset (positive) the traced shape, in pixels. */
+  offset: z.coerce.number().optional(),
 });