docs: Improve Rustdoc for compression engine, delete mode, and parser modules

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

PLAN.md

@@ -213,15 +213,15 @@ Private helpers (e.g., internal `fn` without `pub`) are not flagged, as they don
     - `new()` function: Partial.
     - Impl `FilterPlugin` methods: No docs.
 
-36. **src/compression_engine.rs**
+36. **src/compression_engine.rs** [DONE]
     - `CompressionType` enum: No doc.
     - `CompressionEngine` trait: Partial.
     - Functions (`get_compression_engine`, `default_compression_type`): Partial.
 
-37. **src/modes/delete.rs**
+37. **src/modes/delete.rs** [DONE]
     - `mode_delete()` function: Partial.
 
-38. **src/parser/mod.rs**
+38. **src/parser/mod.rs** [DONE]
     - Re-exports: No docs needed.
 
 39. **src/meta_plugin/hostname.rs**