feat: build a gallery scene for each feature, replayed from its sign

[adamico]

The Gallery pack builds a scene for every feature in the README, not just the Void Dragon's, and each scene's sign rebuilds the scene when clicked so it can be watched again.

Scenes

voidworks:gallery/build lays out 12 scenes in four rows of three. A single long row ran past the simulation distance, so the far scenes never ticked.

Scene	Shows
Void Dragon	Carries motes between two Void Wells
Overworld	A Displacer turns a stone mound into Voidstone, its stone collected in a chest
The End / The Nether	A working Displacer when built in its own dimension; elsewhere an item display over a pedestal, since a Displacer outside its dimension drops as an item
Density	Four one-harvest Displacers, each with its fed block framed on its front, yielding 1, 4, 16 and 64 motes
Networks	Motes crawling along one Voidstone line beside an empty one
Charging / Emptying	A Siphon in a chest by a full patch of Voidstone, or one already charged by a bare patch
Dormant	A Mutation Chamber pays its cost from a line of motes and wakes
Mutation / Unmaking / Crucible	The machine working from a hopper on its side, so the hopper doesn't hide what it shows on top

How it works

• Every scene is built by shared gallery/clear and gallery/sign functions.
• A sign's click event runs execute positioned ~-14 ~ ~ run function voidworks:gallery/<scene>. The command runs at the sign's centre, so it steps back to the plot's corner.
• Anyone who clicks a sign can rebuild that scene: sign commands run with operator permission whoever clicks. All a rebuild does is clear and refill its own plot.
• The clear removes loose items after its fills, because removing the floor pops the sign before the fill reaches it.
• Docs: GLOSSARY.md and ADR 0008 say a scene is replayed from its sign. The README lists the Void Well and the Void Dragon, drops a duplicated Unmaking Chamber line, and describes the gallery.

Tests

GalleryTests now:

• forceloads the gallery's chunks;
• checks every scene's key block, the item displays, and the Emptying scene's charged Siphon;
• checks that Density yields each Density's motes, that the Dormant Chamber wakes, and that clicking a sign rebuilds its scene with nothing left loose.

sh ./gradlew build and the whole game-test suite pass locally on the rebased head.

Known issues, filed separately

• A Displacer drops its last harvest's output when it turns to Voidstone #97: a Displacer drops its last harvest's output when it turns to Voidstone, so the Overworld and Density scenes leave a stone lying.
• An End Displacer on the ground roofs itself over with Voidstone #100: an End Displacer on the ground roofs itself over with Voidstone.

Closes #77

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