Implement a Dynamic Array (Vector) From Scratch

Question

## Problem

Implement a growable array (like `std::vector`) without using built-in dynamic containers: `push_back`, `at`, `size`, amortized O(1) growth.

## Example
```
v.push_back(1); v.push_back(2); v.at(1) -> 2
```

## Constraints
- Manage capacity doubling and copies yourself

## What NVIDIA looks for
They want you to build from scratch, not call library functions. Discuss amortized analysis and move semantics.