CRITICAL: No crash cleanup handlers - data loss on segfault

## Why is this an issue?

No atexit() or SIGSEGV handler to:
- Flush WAL
- Persist dirty pages
- Save catalog

Crash = data loss for all in-flight transactions.

## What is causing it?

Missing crash cleanup.

## How can it be solved?

Add atexit and signal handlers.

## Category
- [ ] Small
- [ ] Medium
- [ ] Large
- [x] Critical

## Severity
- [ ] Low
- [ ] Medium
- [ ] High
- [x] Critical