BedrockMappingsMerger

Merges N rspm_geyser_mappings.json files (one per backend) into a single Geyser custom-mappings JSON for the proxy.

Input shape (produced by com.magmaguy.resourcepackmanager.bedrock.generic.GenericGeyserMappingBuilder.merge):

{
  "format_version": 2,
  "items": {
    "minecraft:base_item": [definition, ...],
    ...
  }
}

Merge rules:

• Union of all base-item keys.
• For shared keys, concatenate the per-backend definition arrays.
• Dedup definitions by bedrock_identifier — on duplicate, last writer wins with a warn.
• Preserve format_version from the first input (they should all match).

Pure JDK + Gson. No Bukkit / Velocity / Geyser API.