Support mcstructure format version 2

[HashimTheArab]

Summary

• write .mcstructure files using format version 2 and NBT int-array block layers
• omit the optional extra block layer when every entry is -1
• continue reading version 1 list-based files and upgrade them when written
• validate version-specific block layer counts and tag representations
• modernise the module for Go 1.26 and current Dragonfly, worldupgrader, and gophertunnel releases

Evidence

Minecraft Preview 26.50.24 documents the v2 format: https://feedback.minecraft.net/hc/en-us/articles/47907593889677-Minecraft-Beta-Preview-26-50-24

Lens verification against client 1.26.50.26 found the matching read, write, and format-bound routines at RVAs 0x79f7050, 0x79f9cb0, and 0x79f7430. The exact-version implementation accepts one or two v2 layers, reads them as TAG_IntArray, and writes the second layer only when it contains a non--1 index. BDS 1.26.50.25 independently showed the same behavior.

Validation

• read a shipped vanilla v1 bastion/mobs/hoglin.mcstructure and rewrote it as v2 with one TAG_IntArray layer
• round-tripped a newly written one-layer v2 structure
• round-tripped a newly written two-layer v2 structure with both layers encoded as TAG_IntArray
• go test ./... -count=1 (builds successfully; repository has no test files)
• go vet ./...
• git diff --check