feat: add 10 emulator profiles (119-series batch 3)

arduous (Arduboy AVR sim, no BIOS), boom3 (Doom 3, game data only),
directxbox (Xbox, mcpx + cromwell), doukutsu_rs (Cave Story, no BIOS),
meteor (GBA, full HLE), nxengine (Cave Story, Doukutsu.exe + data/),
tgbdual (Game Boy link, HLE), tic80 (fantasy console),
virtualxt (IBM XT, open BIOS embedded), sdlpal (Sword & Fairy, 13 .mkf)

125 total profiles.

emulators/nxengine.yml

@@ -0,0 +1,64 @@
+emulator: NXEngine
+type: libretro
+source: "https://github.com/libretro/nxengine-libretro"
+systems: [cave-story]
+
+notes: |
+  NXEngine is an open-source reimplementation of the Cave Story (Doukutsu
+  Monogatari) engine by Studio Pixel. It is not an emulator but a source port
+  that loads the original freeware game data directly.
+
+  The core requires the original freeware Cave Story distribution placed in
+  {system_dir}/nxengine/. When launched without a content file, it looks for
+  Doukutsu.exe at that path (libretro.cpp:254-259). When launched with a .exe
+  content file, it uses the parent directory of that file instead
+  (libretro.cpp:244).
+
+  At startup (main.cpp:66-90), the core opens Doukutsu.exe and extracts from
+  it: ORG music files (extractorg.c), PXT sound effects (extractpxt.c), stage
+  tile attributes (extractstages.c), bitmap graphics, and the wavetable
+  (cachefiles.c:484). These are kept in memory, not written to disk.
+
+  The data/ directory must exist alongside Doukutsu.exe with the full game
+  asset tree: 30 root-level files (sprites, backgrounds, script tables),
+  36 NPC sprite sheets in data/Npc/, and 333 stage files in data/Stage/
+  (maps .pxm, tile attributes .pxa, entity lists .pxe, scripts .tsc, and
+  tileset images via Prt*.pbm). The core verifies data/ exists by checking
+  for data/npc.tbl (main.cpp:47-63).
+
+  The core also reads endpic/ bitmaps for ending sequences, extracted from
+  Doukutsu.exe at runtime (cachefiles.c:460-484).
+
+  All required files ship in the datafiles/ directory of the libretro repo
+  itself, since Cave Story is freeware. No separate BIOS or firmware is
+  needed. The entire freeware distribution is the "system" requirement.
+
+  sprites.sif has a compiled-in fallback (sprites_sif.h) and is the only
+  data file that can be missing without a fatal error (cachefiles.c:515-521).
+
+  Valid content extension: .exe (retro_get_system_info, libretro.cpp:96).
+
+files:
+  - name: "Doukutsu.exe"
+    system: cave-story
+    description: "Cave Story freeware executable (data source for music, sfx, bitmaps, wavetable)"
+    required: true
+    size: 1478656
+    md5: "38695d3d69d7a0ada8178072dad4c58b"
+    sha1: "bb2d0441e073da9c584f23c2ad8c7ab8aac293bf"
+    source_ref: "main.cpp:77-78 (opened for extraction), libretro.cpp:258 (existence check)"
+    notes: "Placed in system/nxengine/. The core extracts ORG music, PXT sounds, stage tile attributes, BMP graphics, and wavetable.dat from this binary at each launch."
+
+  - name: "data/npc.tbl"
+    system: cave-story
+    description: "NPC attribute table (entity behavior flags, HP, damage, display rect offsets)"
+    required: true
+    source_ref: "main.cpp:50 (existence check for data/ directory validation)"
+    notes: "Located in system/nxengine/data/. The core uses this file to verify the data directory is present."
+
+  - name: "data/"
+    system: cave-story
+    description: "Full game asset directory tree (399 files: sprites, NPC sheets, stage maps, scripts, backgrounds)"
+    required: true
+    source_ref: "cachefiles.c:38-480 (complete file list loaded at init)"
+    notes: "Must contain root assets (Arms.pbm, MyChar.pbm, etc.), Npc/ (36 sprite sheets), and Stage/ (333 map/script/tileset files). All files from the original freeware release."