v0.4.0

Release version 0.4.0:

• Implement zero-allocation cursor-stack range traversal in Range: avoids heap allocations and root-to-leaf re-traversals on adjacent key iterations, reducing 100-item range scan from 1.56 µs to 560 ns (178.5M items/sec).
• Fast-path descent for start-of-keyspace range queries.
• Update benchmark tables with 560 ns range scan measurements.