feat: emulator mode field, archive ZX81 standalone ROMs

emulator profiles support mode: standalone | libretro | both.
cross_reference.py skips standalone-only files for libretro platforms.
81.yml: type standalone + libretro, upstream ref added, files listed
with mode: standalone and source_refs to both codebases.
bios/Sinclair/ZX 81/: zx81.rom (8K) and dkchr.rom (4K) archived.

emulators/81.yml

@@ -1,6 +1,7 @@
 emulator: EightyOne
-type: libretro
+type: standalone + libretro
 source: "https://github.com/libretro/81-libretro"
+upstream: "https://github.com/charlierobson/EightyOne"
 profiled_date: "2026-03-18"
 core_version: "1.0a"
 display_name: "Sinclair - ZX 81 (EightyOne)"
@@ -10,43 +11,34 @@ systems:
   - sinclair-zx81
 
 notes: |
-  EightyOne is a Sinclair ZX81 emulator ported to libretro. Also supports
-  ZX80, Jupiter ACE, TS1500, Lambda, R470, TK85, and ZX97LE machines, but
-  the libretro port hardcodes MACHINEZX81 in retro_load_game (libretro.cpp:393).
+  Libretro core embeds zx81.rom and dkchr.rom as C arrays (xxd -i at build).
+  Core never calls RETRO_ENVIRONMENT_GET_SYSTEM_DIRECTORY.
+  Standalone EightyOne loads ROMs from filesystem (cwd/ROM/<filename>).
 
-  ROM loading is in snap.c:487-512. The memory_load() function matches the
-  requested ROM filename against a compiled-in table and copies from embedded
-  byte arrays generated by xxd -i at build time (Makefile.libretro:519-522).
-  Only two ROMs are embedded: zx81.rom (the ZX81 8K BASIC ROM) and dkchr.rom
-  (dK'tronics character ROM). Any other ROM name logs an error and returns 0.
+  Original emulator supports ~15 machines (ZX80, ZX81, Jupiter ACE, TS1500,
+  Lambda, R470, TK85, ZX97LE, Spectrum variants). Libretro port hardcodes
+  MACHINEZX81 — only zx81.rom and dkchr.rom are embedded.
 
-  The core never calls RETRO_ENVIRONMENT_GET_SYSTEM_DIRECTORY, never opens
-  files from the filesystem for ROM data, and need_fullpath is set to false
-  (libretro.cpp:308). All ROM data is baked into the compiled .so/.dll.
+files:
+  # mode: standalone = only needed when running as standalone emulator
+  # mode: libretro = only needed when running as libretro core
+  # mode: both (default if absent) = needed in both modes
 
-  No external BIOS or firmware files are needed.
+  # ref: 81-libretro/81/zx81/snap.c:37,489-496 — embedded as bin_ROM_zx81_rom
+  # ref: EightyOne/Source/zx81/snap.cpp:1219-1279 — open(cwd/ROM/zx81.rom)
+  - name: zx81.rom
+    system: sinclair-zx81
+    required: true
+    mode: standalone
+    size: 8192
+    note: "ZX81 8K BASIC ROM. Embedded in libretro core, external for standalone."
+    source_ref: "EightyOne/Source/zx81/snap.cpp:1264, 81-libretro/81/zx81/snap.c:489"
 
-files: []
-
-analysis:
-  embedded_roms:
-    zx81_rom:
-      variable: bin_ROM_zx81_rom
-      source_file: "bin/ROM/zx81.rom"
-      build_step: "xxd -i -> C array"
-      source_ref: "snap.c:489-498"
-    dkchr_rom:
-      variable: bin_ROM_dkchr_rom
-      source_file: "bin/ROM/dkchr.rom"
-      build_step: "xxd -i -> C array"
-      source_ref: "snap.c:499-508"
-  machine_roms:
-    notes: |
-      eo.c sets CurRom based on machine type. The libretro port forces
-      MACHINEZX81 (eo.c:39-41), so CurRom is always zx81.ROM81 which
-      defaults to "zx81.rom". Other machine ROMs (zx80, ace, ts1500,
-      lambda, zx97le, ringo470, tk85) are referenced but only zx81.rom
-      and dkchr.rom are embedded. Selecting other machines would log
-      "ROM not found" and load nothing.
-  filesystem_access: false
-  system_directory_used: false
+  # ref: 81-libretro/81/zx81/snap.c:38,499-508 — embedded as bin_ROM_dkchr_rom
+  - name: dkchr.rom
+    system: sinclair-zx81
+    required: false
+    mode: standalone
+    size: 4096
+    note: "dK'tronics character ROM. Embedded in libretro core."
+    source_ref: "81-libretro/81/zx81/snap.c:499"