Document and test preview compatibility (#10)

README.md

@@ -77,6 +77,9 @@ view. Without either integration, Rivencraft uses its built-in animated linking
 panel. Clients can set `book_preview.mode` in
 `rivencraft-client.toml` to `AUTO`, `IMMERSIVE_PORTALS`, `PONDER`, or `LEGACY`.
 Unavailable optional renderers safely fall back to `LEGACY`.
+The complete backend matrix, supported combinations, cleanup behavior, and
+known upstream limits are documented in
+[docs/PREVIEW_COMPATIBILITY.md](docs/PREVIEW_COMPATIBILITY.md).
 
 ## Build and run