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>

mod/src/main/kotlin/dev/cobblesurvival/CobbleSurvival.kt

@@ -9,6 +9,7 @@ import dev.cobblesurvival.dev.OreVeins
 import dev.cobblesurvival.ore.BaseLoot
 import dev.cobblesurvival.ore.OreNodes
 import dev.cobblesurvival.permadeath.Permadeath
+import dev.cobblesurvival.machine.MachineShowcase
 import dev.cobblesurvival.machine.Machines
 import dev.cobblesurvival.party.ShoulderPassives
 import dev.cobblesurvival.progress.Gates
@@ -115,6 +116,7 @@ class CobbleSurvival {
             })
             .then(BiomePreview.command())
             .then(OreVeins.command())
+            .then(MachineShowcase.command())
             .then(Commands.literal("zone").executes { context ->
                 val source = context.source
                 val zone = Zones.at(source.level, BlockPos.containing(source.position))