What's Changed

Eliminates runtime.memmove (16% CPU) from the write path by replacing sorted buffer insertion with unsorted append and lazy sorting.

• appendToBuffer switched from sorted insertion (O(B) memmove per insert) to O(1) unsorted append
• Removed eager sorting from flushNode — sorting now only happens at leaf merge time via resolveMessages
• Added linear-scan fallback in bufferMessagesForKey and bufferSlice for reads on unsorted buffers (safe under RLock)
• Batch leaf merge and iterator refactor
• Cross-impl benchmarks now match Google BTree node config (WithBlockSize(63))

Benchmarks (vs v0.3.0)

Read regressions are expected and acceptable — this is a write-optimized B^ε-tree. The write path no longer pays any sorting or memmove cost on insert. Full
comparison in benchmarks/benchstat_v0.3.0...v0.4.0.txt.

Docs

• ADR-004: design rationale, alternatives rejected, invariants