Phase 6: shell preview, core tooltips, machine test world, faster furnace

Adrian (2026-11-02): double the furnace speed; there must be a way in
game to see how a machine is built; a test world with the machines.

- Large Furnace speed doubled (2 + 0.08 x level + 0.02 x SpA).
- Ghost preview: placing a core, using it with an incomplete shell, or
  sneak-using it empty shows every wrong spot as a small glowing block
  display for 10 s (never saved), in the core's orientation, with a
  chat line counting what's missing.
- Core tooltips: what the machine does, what its shell is made of.
- /cobblesurvival machines showcase builds every machine three times
  (working with a Showcase-owned Pokemon, empty, bare core) with signs,
  hoppers, chests and cables; tools/make_machine_world.py makes a
  superflat creative world from it (level.dat via tools/nbtfile.py)
  and --prism installs it as "Machine Test (Phase 6)".
- Self-tests machine_preview_shows_the_shell and
  machine_showcase_builds_every_machine; docs and checklist.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -59,6 +59,12 @@ python3 tools/make_world.py
 ```
 Builds the island world template (`build/world-template`, about 20 minutes: pregeneration, then a pass that waits until every chunk has its ore veins and nodes), runs the same world checks on the whole island plus border fit and End access, and renders `build/island-map.png`. Only needed when the island itself should change. Details in [world/README.md](world/README.md). Run `tools/prism_instance.py` afterwards to put it in Prism as the save "Cobblemon Survival Island".
 
+```bash
+python3 tools/make_machine_world.py --prism
+```
+
+Builds the machine test world (`build/machine-world`, a minute): superflat, creative, cheats on, every Phase 6 machine built three times (working with a Pokémon inside, empty, bare core). `--prism` copies it into the Prism instance as the save "Machine Test (Phase 6)", replacing an older copy.
+
 ```bash
 python3 tools/zone_layout.py
 ```