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/architecture.md

@@ -86,6 +86,7 @@ apps/web ──► apps/proxy ──► packages/tts ──► packages/shared
 | `@hono/cors`       | CORS middleware                           | `apps/proxy`       |
 | `bson`             | BSON deserialization of TTS save files    | `packages/tts`     |
 | `@visioncortex/vtracer` | Raster-to-SVG vectorization (wasm)   | `apps/proxy`       |
+| `clipper-lib`     | Polygon offsetting (inset/outset) for traced shapes | `apps/proxy`       |
 | `sharp`            | Image decoding to RGBA                     | `apps/proxy`       |
 | `svgpath`          | SVG path parsing for traced shapes         | `apps/proxy`       |
 | `cheerio`          | Workshop browse page scraping             | `apps/proxy`       |
@@ -101,9 +102,9 @@ apps/web ──► apps/proxy ──► packages/tts ──► packages/shared
   must stay isomorphic.
 - **`bson` is Node-only** — used by the fetcher and the trace route, not the
   analysis layer.
-- **`@visioncortex/vtracer`, `sharp`, `svgpath` are backend-only** — the trace
-  route lives in `apps/proxy`; they never appear in `packages/extract`, which
-  must stay isomorphic.
+- **`@visioncortex/vtracer`, `sharp`, `svgpath`, `clipper-lib` are
+  backend-only** — the trace route lives in `apps/proxy`; they never appear in
+  `packages/extract`, which must stay isomorphic.
 - **`packages/extract` has zero external runtime deps** — it relies only on
   platform `fetch` / `Blob`, keeping it portable to a future frontend.