feat: add demo world datapack

[Indemnity83]

Summary

Adds a committable Minecraft datapack under demo/ that builds a demo world showcasing the mod's core features. Run /function logistics:build_demo in any flat world with the mod installed to set up all demos automatically.

Demos

• Demo 1 – Basic Transport: Chest A → extractor pipe → stone pipes → Chest B, powered by a Redstone Engine
• Demo 2 – Smart Filtering: Mixed items routed to separate chests via an Item Filter Pipe (requires manual GUI config)
• Demo 3 – Network Logistics: Provider pipe → logistics network → Requester pipe (requires manual GUI config)
• Demo 4 – Autocrafting: Iron ingots + sticks auto-crafted into iron swords on demand via a pre-configured Crafter block
• Demo 5a – Kiln: Hopper-fed RF smelting with a Creative Engine; output piped to chest
• Demo 5b – Macerator: Iron ore ground to iron dust x2 (ore doubling demo); same hopper/pipe layout
• Demo 6 – Laser Quarry: Creative Engine powering a 16×16 quarry; output piped east to a chest

Automated Tests

/function logistics:test/run_all resets and re-runs three automated pass/fail checks via scoreboard:

• Transport (10 s): iron ingots arrive in Chest B
• Kiln (30 s): smelted items arrive in output chest
• Macerator (30 s): iron dust arrives in output chest

Results display on the sidebar scoreboard in real time.

Notes

• Demos are spaced 15 blocks apart along the Z axis (z=10, 25, 40, 55, 70, 85)
• All floating label signs use standing signs facing north so they're readable as you approach
• Datapack targets MC 1.21.1 (pack_format: 48) but the functions are version-agnostic