feat: align gap analysis coherence, add 7 BIOS files, unsourceable field

cross_reference.py: add bios_mode/agnostic/load_from filters, archive
grouping, unsourceable field support. verify.py: case-insensitive
by_name lookup, storage:release in_repo, unsourceable skip, trailing
slash fix. generate_site.py: enriched all_declared, platform-relevant
profile filtering, proper in_repo resolution on emulator pages,
acknowledged gaps section.

New BIOS: delta2.rom (XRoar), tilekey.dat + sprites.sif (NXEngine),
Gram Kracker.ctg + cf7+.ctg + ti-pcard.ctg (ti99sim), desc.dat
(SDLPAL). Profiles: hle_fallback on tilekey.dat/key.txt, unsourceable
on 7 files with source-verified reasons.

emulators/fmsx.yml

@@ -238,6 +238,7 @@ files:
   - name: CARTS.CRC
     required: false
     bundled: false
+    unsourceable: "dead legacy code path, never created or distributed, replaced by CARTS.SHA"
     note: "CRC database for cartridge identification and mapper detection. Tried first, before CARTS.SHA (fMSX/MSX.c:2697)."
 
   - name: CARTS.SHA

emulators/np2kai.yml

@@ -186,6 +186,7 @@ files:
     path: "np2kai/gpib.rom"
     size: 8192
     required: false
+    unsourceable: "never publicly dumped, GP-IB emulation is stub code (source comment: not implemented)"
     note: >
       GP-IB interface BIOS ROM (8 KB). If missing, GP-IB emulation
       is disabled entirely.
@@ -198,6 +199,8 @@ files:
   - name: "key.txt"
     path: "np2kai/key.txt"
     required: false
+    hle_fallback: true
+    unsourceable: "user-created keyboard config, no default distributed in any NP2kai release"
     note: >
       Keyboard remapping configuration (text file). User-created file
       for custom keyboard layout. The core uses built-in defaults if absent.

emulators/nxengine.yml

@@ -81,5 +81,6 @@ files:
     system: cave-story
     description: "Tile attribute lookup table (maps tile codes to collision/behavior attributes)"
     required: false
+    hle_fallback: true
     source_ref: "map.cpp:290-303 (loaded at init, hardcoded default if missing)"
     note: "Not part of the freeware distribution. Generated by the standalone NXEngine extraction tool. The libretro core has hardcoded defaults in map.cpp:30."

emulators/puae.yml

@@ -247,6 +247,7 @@ files:
   - name: "WHDLoad.key"
     system: commodore-amiga
     required: false
+    unsourceable: "per-user signed registration key, never distributed generically, WHDLoad free since v18.2"
     note: "WHDLoad license key. Copied to saves/WHDLoad/L/ for registered WHDLoad use."
     source_ref: "libretro/libretro-core.c:5985-5998"
 

emulators/puae2021.yml

@@ -251,6 +251,7 @@ files:
   - name: "WHDLoad.key"
     system: commodore-amiga
     required: false
+    unsourceable: "per-user signed registration key, never distributed generically, WHDLoad free since v18.2"
     note: "WHDLoad license key. Copied to saves/WHDLoad/L/ for registered WHDLoad use."
     source_ref: "libretro/libretro-core.c:5903-5916"
 

emulators/xroar.yml

@@ -397,5 +397,6 @@ files:
     system: dragon64
     description: "Ikon Ultra Drive Dragonfly ROM 2.3"
     required: false
+    unsourceable: "not yet dumped, only v1.3 publicly available, XRoar support is experimental (#ifdef WANT_EXPERIMENTAL)"
     source_ref: "xroar.c:715 (romlist ikon), ikon.c:151 (default @ikon)"
     note: "Experimental. Ikon Ultra Drive storage interface. Older version: dragonfly-1.3."

emulators/zc210.yml

@@ -138,6 +138,8 @@ files:
 
   - name: Custom.dat
     path: zc210/sfx/Custom.dat
+    category: game_data
     description: user-provided custom SFX replacement
     required: false
+    unsourceable: "user placeholder slot, README says 'rename your own SFX dat file to this name'"
     source_ref: "zelda.cpp:1193-1218, libretro.cpp:148"