docs: Add rustdoc comments for text metadata plugin internals

Co-authored-by: aider (openai/andrew/openrouter/sonoma-sky-alpha) <aider@aider.chat>

PLAN.md

@@ -71,20 +71,20 @@ Private helpers (e.g., internal `fn` without `pub`) are not flagged, as they don
    - `register_filter_plugin()` and `get_available_filter_plugins()` functions: Partial.
    - Overall: Global registry lacks docs.
 
-8. **src/services/item_service.rs**
+8. **src/services/item_service.rs** [DONE]
    - `ItemService` struct: Partial.
    - Many methods (`get_item`, `get_item_content`, `get_item_content_info`, `find_item`, `list_items`, `delete_item`, `save_item`, `save_item_from_mcp`, `get_compression_service`, `get_data_path`): Partial or no docs.
    - `FilteringReader` struct and impl `Read`: No docs.
    - Overall: Extensive but inconsistent docs.
 
-9. **src/config.rs**
+9. **src/config.rs** [DONE]
    - Enums (`ColumnAlignment`, `ContentArrangement`, `TableStyle`, `TableColor`, `TableAttribute`): Partial or no docs.
    - Structs (`TableConfig`, `ColumnConfig`, `ServerConfig`, `CompressionPluginConfig`, `MetaPluginConfig`, `Settings`): Partial.
    - `new()` method on `Settings`: Partial.
    - Helper methods (`get_server_password`, `server_password`, etc.): No docs.
    - Overall: Config structs need better field-level docs.
 
-10. **src/meta_plugin/text.rs**
+10. **src/meta_plugin/text.rs** [DONE]
     - `TextMetaPlugin` struct: No doc.
     - `new()` function: No doc.
     - Many helper functions (`count_text_stats`, `perform_binary_detection`, etc.): No docs.