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.

docs/implementation-plan.md

@@ -241,16 +241,23 @@ Hono server exposing search + fetch.
     headers, which would block three.js loaders in the browser; routing through
     the proxy makes those assets loadable. Only `http(s)` URLs are allowed.
 - `routes/trace.ts`
-  - `GET /trace?url=...&mode=alpha&threshold=128&format=shape` — fetch an
-    image, trace it into a vector shape, and return the result BSON-encoded.
-    `mode` is `alpha` (default), `bw`, or `color`; `format` is `shape`
-    (default) or `svg`. `shape` returns a parsed `{ outline, holes }` polygon
-    matching `@tts/mesh`'s `Shape` interface, ready to extrude.
+  - `GET /trace?url=...&mode=alpha&threshold=128&format=shape&offset=...` —
+    fetch an image, trace it into a vector shape, and return the result
+    BSON-encoded. `mode` is `alpha` (default), `bw`, or `color`; `format` is
+    `shape` (default) or `svg`; `offset` (optional) insets (negative) or
+    outsets (positive) the shape in pixels. `shape` returns a parsed
+    `{ outline, holes }` polygon matching `@tts/mesh`'s `Shape` interface,
+    ready to extrude.
 - `routes/svgShape.ts`
   - `parseSvgShape(svg)` — parse a vtracer SVG into a `TracedShape`
     (`{ outline, holes }`): flatten beziers to polylines, split subpaths into
     rings, classify by winding (CCW outline / CW hole), and assign holes to
     their containing outline.
+  - `offsetShape(shape, delta)` — inset/outset a `TracedShape` via
+    `clipper-lib` (Clipper miter joins); outline and holes offset in opposite
+    directions and are recombined 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.
 - `routes/health.ts`
   - `GET /health` — liveness.
 - `env.ts` — zod validation of `STEAM_API_KEY`, `PORT`.
@@ -309,7 +316,7 @@ proxy API and `packages/extract` directly for analysis.
 | GET    | `/items/:id`        | Full parsed `TTSMod` (`?fileUrl=` skips key) | key* |
 | GET    | `/items/:id/file`   | Raw save bytes, filename from header         | key* |
 | GET    | `/asset?url=`       | CORS-safe proxy for external assets          | —    |
-| GET    | `/trace?url=&mode=&format=` | Trace an image into a vector shape (BSON) | —    |
+| GET    | `/trace?url=&mode=&format=&offset=` | Trace an image into a vector shape (BSON) | —    |
 
 \* `STEAM_API_KEY` is optional; `/items/*` works without it when a `fileUrl`
 query param is supplied.