Copy-on-Write Branch / Snapshot (Practical)

Question

## Problem

Implement copy-on-write branching: branching a dataset shares pages until one branch writes, then copies the page (Neon's instant DB branching).

## Example
```
branch() shares pages; write on branch copies only that page
```

## Constraints
- O(1) branch; copy only on write

## Approach
Reference-counted pages; copy-on-write on first mutation. Discuss space sharing.