A RetroArch store engine for WarpEngine catalogs

The Batocera store reaches one kind of machine. RetroArch reads the same
cartridges on desktop Linux, Windows and macOS, on Android, on the Steam
Deck and on the console ports — the same seven titles, a far larger
audience — so this writes RetroArch's own library format instead:
one .lpl playlist per platform, with box art in the thumbnail folders that
playlist's name points at.

Three things it does differently, each because RetroArch is not one
machine:

- It reads retroarch.cfg for its directories. On the macOS install this
  was developed against, playlist_directory is ~/Documents/RetroArch/
  playlists while cores and thumbnails are under ~/Library/Application
  Support/RetroArch — an engine assuming <base>/playlists writes into the
  void. The leading ':' of a portable install is expanded too.
- It can write for a machine it is not running on (`export`). Android runs
  RetroArch but cannot run Python, so the only way to serve it is to
  render the tree here and copy it over; the same mechanism handles an SD
  card that will be mounted elsewhere.
- It refuses to write while RetroArch is running, because RetroArch holds
  its playlists in memory and writes them back on exit. The Batocera
  engine defers the write instead; here there is no Ports menu launching
  us, so refusing is both simpler and safer.

Decisions worth recording. One playlist per platform, because
default_core_path is a header field — that is what makes every entry start
without a core prompt; the entries themselves stay on DETECT so the file
survives being moved to a machine whose cores live elsewhere. A missing
core is a note, not an error, or the store would never run for anyone the
first time. The playlist name carries the store's name, so we never write
into RetroArch's own Commodore - 64.lpl and never collide with the
official thumbnail packs — and an entry the user added to our playlist
survives a rewrite. Thumbnails must be PNG and one of our covers is a
GIF, so the engine shells out to sips/magick/convert/ffmpeg, and simply
goes without the image if the machine has none of them.

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

config.example.json

@@ -0,0 +1,67 @@
+{
+  "store": {
+    "id": "warp",
+    "name": "WarpEngine Store",
+    "base_url": "https://example.org",
+    "api": {
+      "catalog": "/api/software",
+      "download": "/api/download"
+    }
+  },
+  "paths": {
+    "retroarch_dir": null,
+    "playlists_dir": null,
+    "thumbnails_dir": null,
+    "libretro_dir": null,
+    "content_root": null,
+    "subfolder": "warp",
+    "target_prefix": null
+  },
+  "playlist": {
+    "name_template": "{store} - {label}",
+    "write_crc32": true,
+    "thumbnail_kinds": [
+      "Named_Boxarts",
+      "Named_Titles",
+      "Named_Snaps"
+    ]
+  },
+  "catalog": {
+    "statuses": [
+      "released",
+      "archived"
+    ],
+    "owner_id": null,
+    "only": [],
+    "exclude": []
+  },
+  "platforms": {
+    "c64": {
+      "label": "Commodore 64",
+      "kind": "cartridge",
+      "ext": ".prg",
+      "core": {
+        "file": "vice_x64_libretro",
+        "name": "VICE x64"
+      },
+      "enabled": true
+    },
+    "tic80": {
+      "label": "TIC-80",
+      "kind": "cartridge",
+      "ext": ".tic",
+      "core": {
+        "file": "tic80_libretro",
+        "name": "TIC-80"
+      },
+      "enabled": true
+    }
+  },
+  "behavior": {
+    "prune": true,
+    "timeout": 30,
+    "insecure": false,
+    "refuse_while_running": true,
+    "convert_images": true
+  }
+}