Implement a Simple Memory Allocator

Question

## Problem

Implement a simple `malloc`/`free` over a fixed arena with a free list, splitting and coalescing blocks.

## Example
```
alloc(16); alloc(32); free(first); alloc(16) reuses freed block
```

## Constraints
- Minimize fragmentation

## What Akamai looks for
They value engineers who 'live close to the stack' — understand headers, alignment, and coalescing.