fix: place Dolphin's User folder a platform sets

Dolphin reads its User folder at $XDG_DATA_HOME/dolphin-emu, else
~/.local/share/dolphin-emu (Source/Core/UICommon/UICommon.cpp:449-477).
The profile names both, so a launcher handing that folder places the
layout:

- EmuELEC starts Dolphin with XDG_DATA_HOME=/storage/roms/dolphin
  (dolphinSA/scripts/dolphin.sh:28): its 30 files move from the BIOS
  root to dolphin/dolphin-emu/ of the install tree.
- RetroDECK links User/Triforce to bios/Triforce
  (dolphin/component_prepare.sh:30, 49), where its list already puts
  segaboot.gcm. The rest of the User folder sits in the Flatpak's data
  directory and is named as read outside the BIOS folder.

An entry only the standalone build reads, path and no standalone_path,
now takes that path as its layout path (Load/Firmware/rtl_bt/), and
the pass copying a name the platform declares elsewhere reaches an
unprefixed path: gba_bios.bin lands in dolphin/dolphin-emu/GBA/.

tests/test_launcher_placement.py

@@ -784,5 +784,28 @@ class CemuOnRetroDECK(unittest.TestCase):
         self.assertEqual(otp.get("unplaced"), "outside_bios_folder")
 
 
+class DolphinUserDirectory(unittest.TestCase):
+    """EmuELEC runs Dolphin with XDG_DATA_HOME=/storage/roms/dolphin (dolphin.sh:28),
+    RetroDECK links $XDG_DATA_HOME/dolphin-emu/Triforce to bios/Triforce
+    (dolphin/component_prepare.sh:30, 49)."""
+
+    def test_emuelec_installs_the_user_directory_below_its_roms(self) -> None:
+        view = standalone_view("dolphin", "emuelec")
+        placed = {e["name"]: e["standalone_path"] for e in view["files"] if e.get("layout_path")}
+        self.assertEqual(placed["segaboot.gcm"], "dolphin/dolphin-emu/Triforce/segaboot.gcm")
+        # A standalone-only entry's path is its place in the User directory.
+        self.assertEqual(
+            placed["rtl8723a_fw.bin"], "dolphin/dolphin-emu/Load/Firmware/rtl_bt/rtl8723a_fw.bin"
+        )
+        self.assertTrue(all(e.get("unprefixed") for e in view["files"] if e.get("layout_path")))
+
+    def test_retrodeck_places_triforce_alone(self) -> None:
+        view = standalone_view("dolphin", "retrodeck")
+        placed = {e["name"]: e["standalone_path"] for e in view["files"] if e.get("layout_path")}
+        self.assertEqual(placed, {"segaboot.gcm": "Triforce/segaboot.gcm"})
+        (ipl, *_) = [e for e in view["files"] if e["name"] == "IPL.bin"]
+        self.assertEqual(ipl.get("unplaced"), "outside_bios_folder")
+
+
 if __name__ == "__main__":
     unittest.main()