docs: update architecture and plan for tree inspector UI

Record the viewer-registry decision and refresh the extract/web
descriptions and repo layout.

docs/architecture.md

@@ -47,8 +47,8 @@ apps/web ──► apps/proxy ──► packages/tts ──► packages/shared
 
 - **`apps/web` → `apps/proxy`** — calls `/search`, `/items/:id`, and
   `/items/:id/file` over HTTP.
-- **`apps/web` → `packages/extract`** — uses `flattenObjects` / `collectRefs`
-  to analyze a loaded `TTSMod` in the browser.
+- **`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
   item requests.
 - **`apps/proxy` → `packages/shared`** — uses shared types and zod schemas for