Add Cherry Grove Sanctuary starter island blueprint

Introduces a third overworld starter option themed around cherry
blossoms and 1.21 blocks. Features a hidden tuff-brick sanctum
accessed via a discoverable oak trapdoor, lit by a copper bulb, with
a crafter and decorated pot as secondary containers.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: tastybento