Give the store home its own name per engine

The desktop engine shares this store root, and both keyed their home by the store
id alone — so installing both from the same publisher meant one config.json and
one state.json between them, and the second install silently adopted the first
one's state. Caught by installing the two side by side.

The home is now `<id>-retroarch`. The installer moves an existing `<id>` home on
its way past, so an install from yesterday keeps its state and its games; the
uninstaller accepts either name. The launcher was already `<id>-retroarch-store`,
so nothing the user types changes.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

README.md

@@ -77,10 +77,14 @@ STORE_CONFIG=./my-config.json ./install.sh
 ```
 
 It reads `store.id` from the config, installs the engine, the shared core and the
-config into `~/.local/share/warp-engine-store/<id>/`, writes a
+config into `~/.local/share/warp-engine-store/<id>-retroarch/`, writes a
 `<id>-retroarch-store` launcher into `~/.local/bin`, prints the paths it resolved
 and runs the first sync.
 
+The store home is `<id>-retroarch`, not `<id>`: the desktop store engine shares
+this root, and a shared home would mean a shared `config.json` and `state.json`.
+An install from before this change is moved on the next run of the installer.
+
 | Environment variable | Meaning |
 |---|---|
 | `STORE_CONFIG` | **required** — path or URL of the store's `config.json` |
@@ -159,7 +163,7 @@ curl -fsSL https://git.teletypegames.org/engines/warpstore/raw/branch/master/uni
 ## Where things land
 
 ```
-~/.local/share/warp-engine-store/example/
+~/.local/share/warp-engine-store/example-retroarch/
 ├── retroarch_store.py, warpstore.py, config.json
 └── state.json, catalog.json