docs: add research domains and resource registry

- add local external resources registry
- add research domain contract and per-repository research domains
- add raw fact snapshot governance and validation scripts

Validation:
- make test
- git diff --check HEAD~1..HEAD

Note:
- welcome workflow failed because its action input names are stale; core content CI passed.

scripts/check-doc-structure.py

@@ -54,9 +54,22 @@ def should_skip(path: Path) -> bool:
     rel = path.relative_to(ROOT)
     if any(part in SKIP_PARTS for part in rel.parts):
         return True
+    if is_research_repository_snapshot(rel):
+        return True
     return any(rel == prefix or prefix in rel.parents for prefix in SKIP_PREFIXES)
 
 
+def is_research_repository_snapshot(rel: Path) -> bool:
+    parts = rel.parts
+    return (
+        len(parts) >= 5
+        and parts[0] == "docs"
+        and parts[1] == "research"
+        and parts[3] == "raw"
+        and parts[4] == "repository"
+    )
+
+
 def duplicate_manual_anchors(path: Path) -> list[str]:
     text = strip_fenced_code(path.read_text(encoding="utf-8", errors="ignore"))
     seen: dict[str, int] = {}