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>

CLAUDE.md

@@ -55,6 +55,7 @@ Typical query flow: `search_types` → `get_class_overview` → `get_member` (~3
 - Deprecated functions excluded from output (`not f.is_deprecated` in visibility filter).
 - Enums with no value descriptions use compact inline format (`Values: A, B, C`) instead of a table.
 - Placeholder descriptions (`------//`) filtered by `_PLACEHOLDER_RE` in `_fn_body()`.
+- Headers with no documented content: `render_header()` returns `""`, `generate.py` skips writing the file and filters those types from `type-index.txt` to avoid dead entries.
 
 ## Critical Gotchas