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/src/routes/trace.ts

@@ -3,7 +3,7 @@ import { serialize } from 'bson';
 import sharp from 'sharp';
 import { createRequire } from 'module';
 import { traceRequestSchema, type TraceResult } from '@tts/shared';
-import { parseSvgShape } from './svgShape.js';
+import { offsetShape, parseSvgShape } from './svgShape.js';
 
 // vtracer is a CommonJS package; load it via require so the wasm initializes
 // with the correct `__dirname`.
@@ -36,7 +36,8 @@ app.get('/', async (c) => {
   if (!parsed.success) {
     return c.json({ error: parsed.error.issues[0]?.message }, 400);
   }
-  const { url, mode, threshold, format, simplify, maxColors } = parsed.data;
+  const { url, mode, threshold, format, simplify, maxColors, offset } =
+    parsed.data;
 
   // Only allow http(s) to avoid SSRF via file://, etc.
   const parsedUrl = new URL(url);
@@ -95,7 +96,12 @@ app.get('/', async (c) => {
     svg,
   };
   if (format === 'shape') {
-    result.shape = parseSvgShape(svg);
+    let shape = parseSvgShape(svg);
+    if (offset !== undefined && offset !== 0) {
+      shape = offsetShape(shape, offset);
+      result.offset = offset;
+    }
+    result.shape = shape;
   }
 
   return new Response(Buffer.from(serialize(result)), {