docs: Add comprehensive rustdoc for meta_plugin/magic.rs, and update PLAN.md

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

PLAN.md

@@ -91,30 +91,30 @@ Private helpers (e.g., internal `fn` without `pub`) are not flagged, as they don
     - Impl `MetaPlugin` methods: No docs.
     - Overall: Complex logic with no rustdoc.
 
-11. **src/compression_engine/program.rs**
+11. **src/compression_engine/program.rs** [DONE]
     - `ProgramReader` and `ProgramWriter` structs: Partial (missing full impl docs).
     - `CompressionEngineProgram` struct: Partial.
     - `new()` function: Partial.
     - Impl `CompressionEngine` methods: Partial.
     - Overall: Good but incomplete for trait methods.
 
-12. **src/meta_plugin/read_time.rs**
+12. **src/meta_plugin/read_time.rs** [DONE]
     - `ReadTimeMetaPlugin` struct: No doc.
     - `new()` function: Partial.
     - Impl `MetaPlugin` methods: No docs.
     - Overall: Similar to read_rate.rs.
 
-13. **src/modes/server/api/common.rs**
+13. **src/modes/server/api/common.rs** [DONE]
     - `ResponseBuilder` struct: No doc.
     - `json()` and `binary()` methods: No docs.
     - Overall: Utility without docs.
 
-14. **src/compression_engine/lz4.rs**
+14. **src/compression_engine/lz4.rs** [DONE]
     - `CompressionEngineLZ4` struct: No doc.
     - `new()` function: No doc.
     - Impl `CompressionEngine` methods: No docs.
 
-15. **src/meta_plugin/magic.rs** (appears to be a duplicate/old version of magic_file.rs)
+15. **src/meta_plugin/magic.rs** [DONE]
     - `MagicFileMetaPlugin` struct: No doc.
     - `new()` function: Partial.
     - Helper functions (`get_magic_result`, `process_magic_types`): No docs.