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/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.