docs: document image tracing endpoint

docs/architecture.md

@@ -46,7 +46,8 @@ apps/web ──► apps/proxy ──► packages/tts ──► packages/shared
 ### Edges
 
 - **`apps/web` → `apps/proxy`** — calls `/search`, `/items/:id`,
-  `/items/:id/file`, and `/asset` (CORS-safe asset proxy) over HTTP.
+  `/items/:id/file`, `/asset` (CORS-safe asset proxy), and `/trace`
+  (image → vector shape) over HTTP.
 - **`apps/web` → `packages/extract`** — uses `buildTree` / `collectRefs`
   to analyze a loaded `TTSMod` in the browser (tree sidebar + asset refs).
 - **`apps/proxy` → `packages/tts`** — calls `fetchMod` / `getFileName` to serve
@@ -80,6 +81,9 @@ apps/web ──► apps/proxy ──► packages/tts ──► packages/shared
 | `@hono/node-server`| Node adapter for Hono                     | `apps/proxy`       |
 | `@hono/cors`       | CORS middleware                           | `apps/proxy`       |
 | `bson`             | BSON deserialization of TTS save files    | `packages/tts`     |
+| `@visioncortex/vtracer` | Raster-to-SVG vectorization (wasm)   | `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`       |
 | `zod`              | Runtime validation                        | `apps/proxy`, `packages/shared` |
 | `three`            | 3D rendering                              | `apps/web`         |
@@ -91,7 +95,11 @@ apps/web ──► apps/proxy ──► packages/tts ──► packages/shared
 
 - **`cheerio` is backend-only** — it never appears in `packages/extract`, which
   must stay isomorphic.
-- **`bson` is Node-only** — used by the fetcher, not the analysis layer.
+- **`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.
 - **`packages/extract` has zero external runtime deps** — it relies only on
   platform `fetch` / `Blob`, keeping it portable to a future frontend.
 
@@ -107,7 +115,7 @@ apps/web ──► apps/proxy ──► packages/tts ──► packages/shared
 
 - The proxy runs as a single Node process via `@hono/node-server`.
 - `apps/web` is a static Vite build served separately; during development it
-  proxies `/search`, `/items`, `/health`, and `/asset` to the proxy.
+  proxies `/search`, `/items`, `/health`, `/asset`, and `/trace` to the proxy.
 - `packages/extract` is published/consumed as a plain ESM module usable from a
   browser bundle or Node.
 - No shared state between requests; each request fetches fresh.