Tailstech rain rarity pass

Based on: simpleclouds-0.7.3+1.21.1-tailstech-rarity-clean

Goal:
- Keep the sky populated with clouds.
- Make actual rain/thunder genuinely occasional instead of near-constant.

Changes:
- Precipitation spawning weights:
  cumulonimbus 1 -> 1
  nimbostratus 2 -> 1
  stratus 3 -> 1
- Fair-weather weights:
  cumulus 16 -> 48
  stratocumulus 10 -> 30
  small_cumulus 14 -> 42
  itty_bitty 14 -> 42

Weighted precipitation share changes from 6/60 = 10% to 3/165 ~= 1.82%.

Rain/thunder formations also have substantially shorter lifetimes and smaller radii,
so even when one exists it is less likely to dominate the player's weather for ages.

No cloud type definitions, shaders, rendering code, or general formation count/interval
were changed.
