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.

scripts/generate_site.py

@@ -1709,6 +1709,35 @@ def generate_gap_analysis(
                 )
             lines.append("")
 
+    # ---- Section 4: Acknowledged gaps (unsourceable files) ----
+
+    all_unsourceable: list[dict] = []
+    for emu_name, data in sorted(report.items()):
+        for u in data.get("unsourceable", []):
+            all_unsourceable.append({
+                "name": u["name"],
+                "emulator": data["emulator"],
+                "reason": u["reason"],
+                "source_ref": u.get("source_ref", ""),
+            })
+
+    if all_unsourceable:
+        lines.extend([
+            "## Acknowledged Gaps",
+            "",
+            f"{len(all_unsourceable)} files documented as unsourceable "
+            "(verified from source code).",
+            "",
+            "| File | Emulator | Reason | Source ref |",
+            "|------|----------|--------|-----------|",
+        ])
+        for u in sorted(all_unsourceable, key=lambda x: x["name"]):
+            lines.append(
+                f"| `{u['name']}` | {u['emulator']} | {u['reason']} "
+                f"| {u['source_ref']} |"
+            )
+        lines.append("")
+
     lines.extend(["", f'<div class="rb-timestamp">Generated on {_timestamp()}.</div>'])
     return "\n".join(lines) + "\n"