refactor: integrate digest functionality into meta plugins and remove digest_engine module

Co-authored-by: aider (openai/andrew.openrouter.qwen.qwen3-coder) <aider@aider.chat>

src/modes/update.rs

@@ -4,7 +4,7 @@ use std::str::FromStr;
 
 use crate::compression_engine::{CompressionType, get_compression_engine};
 use crate::db;
-use crate::digest_engine;
+use crate::meta_plugin;
 use crate::modes::common::{cmd_args_digest_type, get_digest_type_meta, store_item_digest_value};
 use clap::Command;
 use clap::error::ErrorKind;
@@ -78,7 +78,7 @@ pub fn mode_update(
             debug!("MAIN: Updating stream size of {:?}", item_path);
 
             // Create and initialize digest engine
-            let mut digest_engine = digest_engine::get_digest_engine(digest_type.clone());
+            let mut digest_engine = meta_plugin::get_meta_plugin(digest_type.clone());
 
             // Read file content and update digest
             let mut reader = compression_engine.open(item_path)?;