Concurrency: Document-Level Locking / MVCC Simulation

Question

## Problem

Simulate document-level concurrency: multiple writers update different documents concurrently without blocking; write-write conflicts on the same doc retry.

## Example
```
writers on different docs proceed; same-doc conflict -> retry
```

## Constraints
- Snapshot isolation semantics

## What MongoDB looks for
Grounded in WiredTiger's MVCC + document-level locking. Reason about snapshot isolation and conflict retries.