feat: add native zstd compression plugin and deduplicate shared compression/meta utilities

- Add zstd crate (v0.13) with native Rust compression engine (level 3)
- Gate behind 'zstd' feature flag, fall back to program-based when disabled
- Extract CompressionService::decompressing_reader/compressing_writer with zstd support
- Extract MetaService::with_collector() to eliminate Arc<Mutex<Vec>> boilerplate
- Extract read_with_bounds() helper for skip+read pattern
- Add input validation for mutually exclusive --id and --tags flags
- Add zstd round-trip tests

src/modes/client/save.rs

@@ -3,6 +3,7 @@ use crate::compression_engine::CompressionType;
 use crate::config::Settings;
 use crate::meta_plugin::SaveMetaFn;
 use crate::modes::common::settings_compression_type;
+use crate::services::compression_service::CompressionService;
 use crate::services::meta_service::MetaService;
 use anyhow::Result;
 use clap::Command;
@@ -73,15 +74,8 @@ pub fn mode(
         meta_service.initialize_plugins(&mut plugins);
 
         // Wrap pipe writer with appropriate compression
-        let mut compressor: Box<dyn Write> = match compression_type_clone {
-            CompressionType::GZip => {
-                use flate2::Compression;
-                use flate2::write::GzEncoder;
-                Box::new(GzEncoder::new(pipe_writer, Compression::default()))
-            }
-            CompressionType::LZ4 => Box::new(lz4_flex::frame::FrameEncoder::new(pipe_writer)),
-            _ => Box::new(pipe_writer),
-        };
+        let mut compressor: Box<dyn Write> =
+            CompressionService::compressing_writer(Box::new(pipe_writer), &compression_type_clone);
 
         loop {
             let n = stdin_lock.read(&mut buffer)?;