perf: switch embedding compression to Zstd-3 + remove redundant shuffle call

- Use Zstd level 3 instead of deflate(1) for embedding dataset compression.
  Auto-shuffle (already the default since the TDT pre-filter commit) is now
  the only shuffle needed — the explicit .with_shuffle() call was redundant.
- Benchmark: save_without_wal_single improves 67 → 61 µs (-9%).

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

crates/clawhdf5-agent/src/schema.rs

@@ -83,14 +83,15 @@ fn build_memory_group(
             let rows_per_chunk = (target_chunk_bytes / (d * 4)).max(1).min(n);
             ds.with_chunks(&[rows_per_chunk, d]);
 
-            // Compression: shuffle + deflate for embeddings when enabled
+            // Compression: Zstd for embeddings — faster than deflate at same ratio.
+            // Shuffle is applied automatically (auto-shuffle pre-filter).
             if config.compression {
                 let level = if config.compression_level > 0 {
-                    config.compression_level
+                    config.compression_level.min(22)
                 } else {
-                    1 // fast default for embeddings
+                    3 // Zstd level 3: fast + good ratio for f32 embeddings
                 };
-                ds.with_shuffle().with_deflate(level);
+                ds.with_zstd(level);
             }
         }