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/common/mod.rs

@@ -26,3 +26,59 @@ pub fn stream_copy<R: std::io::Read + ?Sized>(
     }
     Ok(())
 }
+
+/// Reads content from a reader with offset and length bounds.
+///
+/// Skips `offset` bytes from the reader, then reads up to `length` bytes
+/// (or all remaining if `length` is 0). Uses PIPESIZE buffers throughout.
+///
+/// # Arguments
+///
+/// * `reader` - The source reader positioned at the start.
+/// * `offset` - Number of bytes to skip before reading.
+/// * `length` - Maximum bytes to read (0 = read all remaining).
+/// * `content_len` - Total content size (used to cap skip/read amounts).
+///
+/// # Returns
+///
+/// A `Vec<u8>` containing the requested byte range.
+pub fn read_with_bounds<R: std::io::Read>(
+    reader: &mut R,
+    offset: u64,
+    length: u64,
+    content_len: u64,
+) -> std::io::Result<Vec<u8>> {
+    // Skip offset bytes
+    let skip = std::cmp::min(offset, content_len);
+    let mut remaining = skip;
+    let mut buf = [0u8; PIPESIZE];
+    while remaining > 0 {
+        let to_read = std::cmp::min(remaining, buf.len() as u64) as usize;
+        match reader.read(&mut buf[..to_read]) {
+            Ok(0) => break,
+            Ok(n) => remaining -= n as u64,
+            Err(e) => return Err(e),
+        }
+    }
+
+    // Read bounded content
+    let max_bytes = if length > 0 {
+        std::cmp::min(length, content_len.saturating_sub(offset))
+    } else {
+        content_len.saturating_sub(offset)
+    };
+    let mut result = Vec::with_capacity(std::cmp::min(max_bytes, 64 * 1024) as usize);
+    let mut bytes_read = 0u64;
+    while bytes_read < max_bytes {
+        let to_read = std::cmp::min(max_bytes - bytes_read, buf.len() as u64) as usize;
+        match reader.read(&mut buf[..to_read]) {
+            Ok(0) => break,
+            Ok(n) => {
+                result.extend_from_slice(&buf[..n]);
+                bytes_read += n as u64;
+            }
+            Err(e) => return Err(e),
+        }
+    }
+    Ok(result)
+}