Skyveil Cloudlands Pass 5 Notes

Added darker rare rain clouds.
- New block: skyveilcloudlands:rain_cloud
- Rain clouds are full darker cloud blocks so Minecraft lighting can cast more shadow below where lighting updates support it.
- Rain clouds drip water particles from below client-side.
- Rain cloud blocks can slowly fill cauldrons placed directly below them, similar to dripstone water collection but without dripstone/spike blocks.
- Rain cloud generation is rare, isolated, and larger/thicker than normal cloud islands.
- Added commands: /skyveilcloudlands generate_rain_here and /skyveilcloudlands generate_rain_near <count>.

Rain cloud generation remains config-gated and intentionally rare.
