feat: shared groups, scraper subdir prefixes, arcade + emulator profiles

shared groups in _shared.yml: np2kai, keropi, quasi88, kronos, ep128emu
with source references for each subdirectory requirement.

libretro_scraper: CORE_SUBDIR_MAP applies subdirectory prefixes at
generation time (np2kai/, keropi/, dc/). EXTRA_SYSTEMS adds QUASI88.
SYSTEM_SHARED_GROUPS injects includes for kronos/np2kai/keropi.

new BIOS: CPS3 (23 ZIPs), Cannonball OutRun (40 ROMs), PCem PC BIOS
(73 files), VICE Commodore ROMs, Spectrum ZIPs, dc_bios.bin, X1 fonts.

new emulator profiles: redream, melonds_ds, lrps2 with doc vs source
notes. platforms/README.md documents shared groups architecture.

emulators/lrps2.yml

@@ -0,0 +1,30 @@
+# LRPS2 — Sony PlayStation 2 (libretro)
+# ref: libretro/lrps2, docs.libretro.com/library/lrps2
+# Same BIOS as PCSX2 standalone, placed in pcsx2/bios/ subfolder.
+# Also needs GameIndex.yaml for game compatibility database.
+#
+# doc vs source: docs say "no specific filename required" for BIOS, any
+# valid 4 MB PS2 BIOS dump works. Docs do not list GameIndex.yaml as a
+# BIOS file, but the source code loads it from pcsx2/resources/ and it
+# is needed for game-specific patches and compatibility fixes.
+# SCPH-10000 not recommended per docs (lower compatibility).
+emulator: LRPS2
+type: libretro
+cores: [lrps2]
+source: "https://github.com/libretro/lrps2"
+systems:
+  - sony-playstation-2
+
+notes: |
+  Libretro port of PCSX2. Uses the same BIOS files as standalone PCSX2.
+  BIOS must NOT be zipped. Any valid PS2 BIOS works (SCPH-10000 not
+  recommended due to lower compatibility). BIOS goes in pcsx2/bios/.
+  GameIndex.yaml goes in pcsx2/resources/ and is needed for proper
+  game-specific settings (patches, fixes).
+
+files:
+  - name: GameIndex.yaml
+    system: sony-playstation-2
+    required: true
+    note: "game compatibility database, not a BIOS — pcsx2/resources/GameIndex.yaml"
+    source_ref: "pcsx2/GameIndex.yaml"

emulators/melonds_ds.yml

@@ -0,0 +1,55 @@
+# melonDS DS — Nintendo DS/DSi (libretro, newer fork)
+# ref: JesseTG/melonds-ds, docs.libretro.com/library/melonds_ds
+# Prefers files in "melonDS DS/" subfolder. DS mode has HLE fallback.
+# DSi mode requires real BIOS + NAND dump.
+#
+# doc vs source: docs use bios7.bin/bios9.bin (NDS) and dsi_bios7.bin/
+# dsi_bios9.bin (DSi). The older melonDS core uses the same names.
+# Repo stores NDS ARM7/ARM9 as biosdsi7.bin/biosdsi9.bin (DSi dumps that
+# also work for NDS). The md5 in docs (df692a80/a392174e) are for the
+# smaller NDS-only dumps, different from the DSi dumps in the repo.
+emulator: melonDS DS
+type: libretro
+cores: [melonds_ds]
+source: "https://github.com/JesseTG/melonds-ds"
+systems:
+  - nintendo-ds
+  - nintendo-dsi
+
+notes: |
+  Newer fork of melonDS for libretro with DSi support.
+  DS BIOS files are optional (built-in FreeBIOS used as fallback).
+  DSi mode requires all four DSi-specific files.
+  Core searches "melonDS DS/" subfolder first, then system root.
+
+files:
+  # DS mode (optional, FreeBIOS fallback)
+  - name: bios7.bin
+    system: nintendo-ds
+    required: false
+    md5: df692a80a5b1bc90728bc3dfc76cd948
+    note: "NDS ARM7 BIOS"
+  - name: bios9.bin
+    system: nintendo-ds
+    required: false
+    md5: a392174eb3e572fed6447e956bde4b25
+    note: "NDS ARM9 BIOS"
+  - name: firmware.bin
+    system: nintendo-ds
+    required: false
+    sha1: 8497afdd16af92d6887b71582846c40514e9039f
+    md5: 145eaef5bd3037cbc247c213bb3da1b3
+    size: 262144
+  # DSi mode (required)
+  - name: dsi_firmware.bin
+    system: nintendo-dsi
+    required: true
+    sha1: d2a5af338f09c5cbdd5d7628db5b9c075c69b616
+    md5: 8daa89fd280b3e5ec79fbab73ad6684e
+    size: 131072
+  - name: dsi_nand.bin
+    system: nintendo-dsi
+    required: true
+    sha1: b48f44194fe918aaaec5298861479512b581d661
+    md5: dfafb1908da8f527df7a372e649b50be
+    size: 251658304

emulators/redream.yml

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+# Redream — Sega Dreamcast (libretro)
+# ref: libretro/redream, docs.libretro.com/library/redream
+# Uses dc/boot.bin and dc/flash.bin (different names from Flycast)
+# Same BIOS data as Flycast's dc_boot.bin / dc_flash.bin
+#
+# doc vs source: docs list boot.bin md5 e10c53c2f8b90bab96ead2d368858623
+# which matches dc_bios.bin in the repo. Flycast uses dc_boot.bin (same
+# data, 2 MB) while Redream calls it boot.bin. The flash.bin is identical
+# across both cores.
+emulator: Redream
+type: libretro
+source: "https://github.com/libretro/redream"
+systems:
+  - sega-dreamcast
+files:
+  sega-dreamcast:
+    - name: dc_bios.bin
+      destination: dc/boot.bin
+      required: true
+      sha1: 8951d1bb219ab2ff8583033d2119c899cc81f18c
+      md5: e10c53c2f8b90bab96ead2d368858623
+      size: 2097152
+    - name: dc_flash.bin
+      destination: dc/flash.bin
+      required: true
+      sha1: 94d44d7f9529ec1642ba3771ed3c5f756d5bc872
+      md5: 0a93f7940c455905bea6e392dfde92a4
+      size: 131072