Mineshaft expansion

What it adds

Two new mineshaft variants (corrupted and reinforced) that spawn more often and come with heavier monster spawn lists. Loot in any mineshaft chest is richer thanks to added pools (high-tier gear, utility, and explosives).
New biome Порча (Porcha): a corrupted cave-like overworld biome with dripstone, sculk growths, lava pockets, and hostile-heavy spawns (cave spiders, husks, endermen, rare wardens). The biome is injected into the overworld climate so it can appear naturally.
Porcha is eligible for vanilla mineshafts and the new mineshaft variants. Custom structure sets control where the variants appear, tied to biome tags.

Worldgen is data-driven (structure JSON, structure sets, biome tags, biome JSON) under the mod data pack. A built-in resource/data pack descriptor is included for 1.21.1 formats.
The mod reflects into the overworld multi-noise preset to add the Порча biome without replacing vanilla files. If reflection fails, generation falls back to vanilla and logs an error.
Additional mob spawns are injected via Fabric biome modifications for mineshaft biomes and the new biome to raise danger.
Loot upgrades are applied via LootTableEvents.MODIFY to the abandoned mineshaft chest table.

Install the JAR on the client (singleplayer or when connecting to a server with matching worldgen). New worlds will occasionally generate Порча biomes; mineshaft variants can be found in tagged underground biomes (including Порча) and have stronger monster presence and better chests.

Reflection-based biome injection may conflict with other mods that deeply rewrite the overworld preset. If another mod replaces the preset entirely, Порча might not appear.
Biome rarity is modest to keep worlds close to vanilla; adjust structure set frequencies or the climate window in code for denser corruption.
Loot boosts affect all mineshafts, not only the new variants; separating tables would require custom structure pieces.