docs: Add rustdoc for filter_plugin, binary_detection, and lib.rs

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

PLAN.md

@@ -50,21 +50,6 @@ Private helpers (e.g., internal `fn` without `pub`) are not flagged, as they don
    - Impl `MetaPlugin` methods (`is_finalized`, `set_finalized`, `finalize`, `update`, `meta_type`, `outputs`, etc.): No docs.
    - Overall: Lacks docs for the public trait impl.
 
-4. **src/filter_plugin/head.rs** [DONE]
-   - `HeadBytesFilter` and `HeadLinesFilter` structs: No docs.
-   - `new()` functions: Partial.
-   - Impl `FilterPlugin` methods (`filter`, `clone_box`, `options`): No docs.
-   - Overall: No rustdoc at all for public impl.
-
-5. **src/common/binary_detection.rs** [DONE]
-   - `check_binary_content_allowed()` function: Partial (missing examples).
-   - `is_content_binary()` function: Partial.
-   - Overall: Functions lack full error handling docs.
-
-6. **src/lib.rs** [DONE]
-   - Module re-exports and `init_plugins()` function: No docs.
-   - Overall: Library-level docs are minimal.
-
 7. **src/services/filter_service.rs** [DONE]
    - `FilterService` struct: Partial doc.
    - `new()`, `create_filter_chain()`, `filter_data()`, `process_with_filter()` methods: Partial or missing.