fix: anomaly detector z-score and test thread-safety bugs

- EmbeddingAnomalyDetector: score against pre-update stats so outlier
  cannot dilute its own z-score by pulling the mean toward itself.
  Handle zero-variance dimensions explicitly: any meaningful deviation
  from an all-identical training set is quarantined immediately.
- Android concurrent test: add `unsafe impl Sync for SendableHandle`
  so Arc<SendableHandle> satisfies the Send bound required by
  std::thread::spawn (Mutex inside the Handle makes this sound).
- clawhdf5-format/clawhdf5 Cargo.toml: remove fast-deflate from
  default features to allow builds in environments without cmake/c++
  (fast-deflate remains available as an opt-in feature).

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

crates/clawhdf5-format/Cargo.toml

@@ -32,7 +32,7 @@ name = "bench"
 harness = false
 
 [features]
-default = ["std", "checksum", "deflate", "provenance", "fast-deflate", "system-zlib-decompress"]
+default = ["std", "checksum", "deflate", "provenance", "system-zlib-decompress"]
 std = []
 checksum = []
 deflate = ["flate2"]