Incremental Memory Cache Database (Multi-Part OA)

Question

## Problem

HubSpot's OA builds incrementally: start with `set/get`, then add TTL expiry, then transactions/rollback, then a scan/filter query — each part building on the last.

## Example
```
Part 1: set/get; Part 2: set with TTL; Part 3: begin/commit/rollback
```

## Constraints
- Each part adds requirements; keep earlier parts working

## What HubSpot looks for
Catch all edge cases and run tests. Communicate and ask clarifying questions — HubSpot expects you've seen the pattern and tests cleanly.