Stage 1: The Memtable

Every LSM-tree engine starts with a memtable — an in-memory map that absorbs all writes at RAM speed. Reads check it first; deletes are just another kind of write.

Implement createStore() returning an object with:

• set(key, value) — store a value
• get(key) — return the value, or undefined if absent
• del(key) — remove the key
• has(key) — boolean

Sounds trivial — it is, deliberately. Real engines are layers on exactly this core, and every later stage builds on yours. Get the semantics right: overwrites replace, deleted keys read as undefined, and has reflects deletes.