docs: Add Rustdoc to gzip and user meta plugins, mark PLAN.md tasks done

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

PLAN.md

@@ -120,27 +120,27 @@ Private helpers (e.g., internal `fn` without `pub`) are not flagged, as they don
     - Helper functions (`get_magic_result`, `process_magic_types`): No docs.
     - Impl `MetaPlugin` methods: No docs.
 
-16. **src/compression_engine/gzip.rs**
+16. **src/compression_engine/gzip.rs** [DONE]
     - `CompressionEngineGZip` struct: Partial.
     - `new()` function: Partial.
     - `AutoFinishGzEncoder` struct: Partial.
     - Impl `CompressionEngine` methods: Partial.
 
-17. **src/modes/server/common.rs**
+17. **src/modes/server/common.rs** [DONE]
     - Many structs (`ServerConfig`, `AppState`, `ApiResponse<T>`, `ItemInfoListResponse`, etc.): Partial or no docs.
     - Functions (`check_auth`, `logging_middleware`, `create_auth_middleware`): Partial.
     - Overall: API structs need better schema/docs.
 
-18. **src/meta_plugin/user.rs**
+18. **src/meta_plugin/user.rs** [DONE]
     - `UserMetaPlugin` struct: Partial.
     - `new()` function: Partial.
     - Helper functions (`get_current_username`, `get_current_groupname`): No docs.
     - Impl `MetaPlugin` methods: Partial.
 
-19. **src/modes/diff.rs**
+19. **src/modes/diff.rs** [DONE]
     - Functions (`validate_diff_args`, `fetch_and_validate_items`, `setup_diff_paths_and_compression`): No docs.
 
-20. **src/modes/get.rs**
+20. **src/modes/get.rs** [DONE]
     - `mode_get()` function: Partial.
     - `TeeReader` struct and impl `Read`: No docs.