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>

datapacks/core/data/cobblesurvival/machines/heat_core.json

@@ -1,5 +1,5 @@
 {
-  "_comment": "Large Furnace (Adrian, 2026-10-31): a Fire Pokémon in the chamber heats vanilla furnaces, blast furnaces and smokers standing against the outside of the shell; they burn without fuel while they have something to cook, and cook faster. Shape: layers bottom to top, rows north to south, columns west to east, as seen with the core in the south wall; checked in all four rotations. 'C' = the core, '.' = the chamber (air; the Pokémon stands there), '#' = casing, ' ' = anything (under the core, so a hopper can go there), 'W' = a window spot: casing or glass (the middle of each side wall and the top centre). Furnaces heated: 1 + level / levels_per_furnace, at most max. Cooking speed: base + per_level × level + per_special_attack × Special Attack. Placeholder numbers; see docs/balance.md. Read by the companion mod (machine/).",
+  "_comment": "Large Furnace (Adrian, 2026-10-31): a Fire Pokémon in the chamber heats vanilla furnaces, blast furnaces and smokers standing against the outside of the shell; they burn without fuel while they have something to cook, and cook faster. Shape: layers bottom to top, rows north to south, columns west to east, as seen with the core in the south wall; checked in all four rotations. 'C' = the core, '.' = the chamber (air; the Pokémon stands there), '#' = casing, ' ' = anything (under the core, so a hopper can go there), 'W' = a window spot: casing or glass (the middle of each side wall and the top centre). Furnaces heated: 1 + level / levels_per_furnace, at most max. Cooking speed: base + per_level × level + per_special_attack × Special Attack. Speed doubled (Adrian, 2026-11-02). Placeholder numbers; see docs/balance.md. Read by the companion mod (machine/).",
   "core": "cobblesurvival:heat_core",
   "kind": "furnace",
   "types": [
@@ -34,9 +34,9 @@
     "max": 8,
     "levels_per_furnace": 15,
     "speed": {
-      "base": 1.0,
-      "per_level": 0.04,
-      "per_special_attack": 0.01
+      "base": 2.0,
+      "per_level": 0.08,
+      "per_special_attack": 0.02
     }
   }
 }