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/compression_engine/program.rs

@@ -1,4 +1,4 @@
-use anyhow::{Context, Result, anyhow};
+use anyhow::{anyhow, Context, Result};
 use log::*;
 use std::fs::File;
 use std::io::{Read, Write};