Rename fly_thumb_georef to fly_georef

## Problem

`fly_thumb_georef()` is not thumbnail-specific — it warps any image to a ground footprint using GCPs from corners. The same logic applies to full-res scans.

## Proposed Solution

Rename `fly_thumb_georef()` → `fly_georef()`. No deprecation needed (package is pre-release, no external users). Document in NEWS.md and bump version.
