v0.5.2

Added

• Added CheckpointImage::validate() to validate a full exported DB
  checkpoint image before install or archive handoff, not just its header.
• Added KeyScanOutcome and KeyRangeBuilder::visit_with_outcome so callers
  can distinguish prefix-list cache hits from real ART walks without changing
  the stable ScanStats field set.
• Added PrefixCount, Tree::prefix_count, and View::prefix_count for
  bounded DFS-style prefix cardinality checks. Non-zero limits scan at most one
  entry past the limit and report whether the count is exact.
• Added DB::scatter_independent for StateMachine-mode independent single-key
  fan-out across named families. It rejects duplicate (tree, key) pairs and
  applies unrelated writes concurrently through Holt's native per-key paths.

Changed

• Refactored DB::scatter to share the same single-key apply helper as
  scatter_independent, keeping ordered scatter semantics while avoiding a
  second implementation of each operation kind.
• Clarified DB::install_checkpoint as a fresh/wiped-DB install path; Holt does
  not expose online live-DB checkpoint replacement.

Validation

• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --test scan_stats --test scatter --test checkpoint