v0.4.0

Added

• Added route-anchor residency so root and hot prefix anchor blobs can
  stay protected from ordinary leaf eviction under large metadata
  working sets.
• Added blob shape-debt counters and richer runtime telemetry for cache
  hit/miss behavior, WAL queue/write/flush progress, checkpoint debt,
  dirty blobs, route-cache behavior, and admission/eviction decisions.
• Added the nightly validation workflow, soak harness, and expanded
  fuzz/property/CI coverage for reopen, checkpoint, range, atomic, and
  crash-oriented metadata behavior.
• Added KeyPath / KeyPathBuf helpers for constructing byte paths
  without making path semantics part of the storage engine core.

Changed

• Reworked route-cache invalidation and scan eviction so prefix-heavy
  metadata reads disturb the buffer pool less at scale.
• Added TinyLFU admission for file-backed cache pressure and raised the
  default file-backed buffer pool budget to match the intended metadata
  working-set profile.
• Tightened same-size leaf updates, SIMD hot paths, and concurrency gate
  internals.
• Simplified checkpoint barrier code, split BufferManager policy helpers,
  and clarified WAL progress metrics.
• Hid uninitialized blob-buffer allocation from the public API surface.

Fixed

• Fixed atomic batch write isolation so Tree::atomic preflight and apply
  are protected from concurrent foreground writes.
• Kept the release and CI dependency stack current, including
  actions/upload-artifact v7 and rand 0.10 for the root test crate.