v0.3.0 - It bores!

The tunnel bore is functional. Mark a flat area (V, left-click), tap V to exit Mark Mode, then break any marked block: the whole layer mines, drops go straight to your inventory, your pickaxe takes durability, and the selection advances into the face you broke so you can keep boring. Full vanilla tool rules (tier, Fortune/Silk Touch, Unbreaking) and safe stops at bedrock / before the pickaxe breaks. Requires Fabric Loader + Fabric API on MC 1.21.