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/tests/compression/mod.rs

@@ -4,3 +4,5 @@ pub mod gzip_tests;
 pub mod lz4_tests;
 #[cfg(test)]
 pub mod none_tests;
+#[cfg(test)]
+pub mod zstd_tests;