Skip empty doc files and prune dead type-index entries

- render_header() returns "" when a header has no documented content
  (no /** */ comments on any class, property, function, enum, or delegate)
- generate.py skips writing those files and tracks which were written
- type-index.txt is filtered to only include types from written files,
  preventing dead entries that would cause get_class_overview to fail
- Summary line now reports how many files were skipped

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docgen/ue_markdown.py

@@ -361,14 +361,13 @@ def _render_namespace(ns: NamespaceInfo) -> str:
 def render_header(parsed: ParsedHeader,
                   type_index: dict[str, str] = None,
                   current_md: str = "") -> str:
+    """
+    Render a ParsedHeader to Markdown. Returns empty string if the header
+    has no documented content (so callers can skip writing the file).
+    """
     if type_index is None:
         type_index = {}
 
-    lines = []
-    lines.append(f"# `{parsed.filename}`")
-    lines.append(f"**Module**: `{parsed.module_name}`")
-    lines.append("")
-
     sections = []
 
     d_sec = _render_delegates(parsed.delegates)
@@ -400,6 +399,13 @@ def render_header(parsed: ParsedHeader,
                 ff_lines.append(_render_ff_compact(fn))
             sections.append('\n'.join(ff_lines))
 
-    lines.append('\n\n---\n\n'.join(sections))
+    if not sections:
+        return ""
 
+    lines = [
+        f"# `{parsed.filename}`",
+        f"**Module**: `{parsed.module_name}`",
+        "",
+        '\n\n---\n\n'.join(sections),
+    ]
     return '\n'.join(lines)