Add weighted multi-material mixing for log/leaf replacement

[Copilot]

Adds support for mixing multiple replacement materials with configurable percentage weights when trees grow in Nether/End dimensions. Each material slot (global, per-tree, per-gamemode) now accepts a material: weight map instead of a single string.

Config format change

nether:
  logs:
    gravel: 80        # 80% gravel
    netherrack: 20    # 20% netherrack
  leaves:
    glowstone: 70
    soul_sand: 30

  per-tree:
    logs:
      oak:
        gravel: 60
        netherrack: 40

Weight semantics

• = 100% — each material at exact stated probability
• > 100% — weights scaled proportionally, no AIR injected; warning logged on load
• < 100% — remainder maps to AIR; warning logged on load

Implementation

• MaterialWeightedPicker (new) — stateless utility; pickRandom(Map<String,Integer>) handles all three cases above
• Settings — global fields String → Map<String,Integer>; per-tree/per-gamemode Map<String,String> → Map<String,Map<String,Integer>>
• TreeGrowEvent — resolveMaterial() replaced by resolveWeights() returning a weight map; applyWeightedMaterial() picks per-block via the picker
• DimensionalTrees.loadSettings() — validates all weight maps on load, emitting warnings for totals ≠ 100%

Breaking config change: existing single-string values (logs: gravel) must be migrated to the map form (logs: {gravel: 100}).