[V2] In-place value updates

[bravo1goingdark]

Summary

Support in-place value updates instead of rejecting duplicate inserts.

Motivation

Currently, inserting a key that already exists returns DuplicateKey error. For some use cases, you may want to update the value in place without creating a new node.

Current Behavior

sl.insert(b"key", b"value1");
sl.insert(b"key", b"value2"); // Returns false, DuplicateKey error

Desired Behavior

sl.insert(b"key", b"value1");
sl.update(b"key", b"value2"); // Updates value in place, returns old value

Or alternatively, make insert have an update mode:

sl.insert(b"key", b"value1");
sl.insert(b"key", b"value2", UpdateMode::Overwrite); // Updates value

Implementation Notes

The design doc mentions "update value (or insert tombstone version for MVCC)" but we went with MVCC approach (seal + fresh memtable). An in-place update could be added as an alternative.

From design doc

See LOCKFREE_SKIPLIST_DESIGN.md lines 191-192

Priority: low