[FEATURE] Range delete — delete all keys in a range (RocksDB DeleteRange equivalent)

[ElioNeto]

Description

Currently ApexStore only supports single-key delete(). There is no way to atomically delete all keys within a range [start, end). Competitive LSM engines (RocksDB, LevelDB, Sled) support range deletion.

Proposed API

engine.delete_range(start: &[u8], end: &[u8]) -> Result<()>

Implementation

1. Create a RangeTombstone record that marks a range as deleted
2. During compaction, check if keys fall within any active range tombstone
3. During point reads, check range tombstones before returning a value
4. Store range tombstones in memtable and SSTable metadata

Impact

• Enables efficient time-series data pruning
• Enables log compaction (delete old logs by time range)
• Reduces write amplification vs scan+delete loop

Labels

[ElioNeto]

Implemented in commit b5d7171. New delete_range(start, end) API with RangeTombstone struct. Tracks range tombstones in memtable and across compaction. get_cf() and scan_cf() filter out range-tombstoned keys. 5 tests.