feat: name the user soundfont retrobat copies

RetroBat copies bios/scummvm/extra/Roland_SC-88.sf2 to DOSBox Pure and
DOSBox Staging and offers it in the soundfont option
(DosBoxStaging.Generator.cs:246-252, 336-344). No canonical file of
that name exists, and both emulators take any SoundFont
(dosbox_pure_libretro.cpp:1380-1412, fluidsynth.cpp:194-230).

emulators/dosbox-pure-unleashed.yml

@@ -283,3 +283,10 @@ files:
       Any .SF2 or .SF3 in the system directory is listed in the MIDI option.
       DOSBOX.SF2 is the fixed name looked up on the mounted C: drive as a per
       game override.
+
+  - name: "Roland_SC-88.sf2"
+    path: "system/Roland_SC-88.sf2"
+    description: "General MIDI SoundFont under a Roland SC-88 name"
+    required: false
+    unsourceable: "user supplied, no canonical file: the system directory scan lists any SoundFont by its own name"
+    source_ref: "upstream dosbox_pure_libretro.cpp:1380-1412"

emulators/dosbox-staging.yml

@@ -696,6 +696,13 @@ files:
       name, and gets its own chorus and reverb parameters in auto mode
     source_ref: "src/midi/fluidsynth.cpp:194-230, src/midi/fluidsynth.cpp:802-808, src/midi/fluidsynth.cpp:430-434, src/midi/fluidsynth.cpp:573-577, src/midi/fluidsynth.cpp:962-987"
 
+  - name: "Roland_SC-88.sf2"
+    path: "soundfonts/Roland_SC-88.sf2"
+    description: "SoundFont under a Roland SC-88 name"
+    required: false
+    unsourceable: "user supplied, no canonical file: the soundfont setting opens any SoundFont it names"
+    source_ref: "src/midi/fluidsynth.cpp:194-230"
+
   - name: "FluidR3_GM.sf2"
     path: "soundfonts/FluidR3_GM.sf2"
     description: "Fluid R3 GM SoundFont by Frank Wen"