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>

docs/design.md

@@ -255,6 +255,8 @@ Adrian's choices: the Large Furnace has a Fire Pokémon heat real furnaces, and
 
 **Rebirth Chamber.** A Lv 100 Pokémon put in starts a long rebirth (placeholder: two hours of server time, also while the chunk is unloaded). Then it is the first stage of its line at Lv 1 and shiny, keeps its nickname, original trainer, held item and every move it learned, gets three of its IVs set to 31 (no IV ever goes down), and keeps its ability; an Ability Patch comes with it, so taking the hidden ability is the player's choice. Taking it out before the end cancels the rebirth.
 
+**Finding out how to build one** (Adrian, 2026-11-02: there must be a way in game). Placing a core, using it while its shell is incomplete, or sneak-using it while empty shows every missing or wrong spot of the shell as a small glowing ghost block for ten seconds (vanilla block displays, so clients need nothing; never saved), in the orientation the core faces, with a chat line counting what's missing. The cores' tooltips say what each machine does and what its shell is made of. The machine test world (`tools/make_machine_world.py`) has every machine built.
+
 **Cores** are crafted (soft gates by material): Heat Core from bricks, iron and a blast furnace; Dynamo Core from copper blocks, redstone and a thunder stone; Synthesis Core from iron and redstone blocks, hoppers and a crafter; Altar Core from obsidian, gold blocks and an echo shard; Rebirth Core from amethyst blocks, diamonds and a shiny stone. Their textures are drawn by `tools/machine_assets.py`.
 
 Self-tests (`MachineTests`): in and out, wrong Pokémon refused, broken shell and broken core (to the PC when the party is full), chunk unload, server restart, and each machine's work (a furnace fed and emptied by hoppers without fuel, a Poke Power cable charged by a Dynamo Core, Magikarp's salmon through a hopper under a gold Synthesis Core, a confirmed sacrifice with a Lv 100 Dragonite's rewards, a rebirth from Charizard to a shiny Lv 1 Charmander with its moves and ability).