feat: show the void taking a harvested block over a second

[adamico]

Closes #98. The design is the Harvesting paragraph in docs/spec/void-energy.md ("Every harvest also shows the void taking the block…"), grilled on the issue; the agent brief is the issue's last comment.

What changed

• Server: a Void Displacer's harvest, a Void Siphon's by hand, and a spent Displacer becoming Voidstone each call VoidTaking.show, which posts a public VoidTakesBlockEvent (position, what it held) on the NeoForge bus, then sends a VoidTaking payload to the players tracking the chunk. The mechanic is unchanged: the position is Voidstone in the harvest's tick, with the same yield, network and Frontier. The 30-particle crumble bursts are gone; the chorus sound stays.
• Client (VoidTakings): what was harvested is drawn over the Voidstone, 1% larger, and eaten away through 12 cumulative stage masks over 20 ticks (TakingStages, TakingMask), each face's masks turned one of 8 ways by its position. A solid is its own model, shedding crumbs from its open faces; a fluid is a cube of its still texture, full bright when it glows (lava); air is the End's starfield. A taking is dropped when its chunk unloads or the player leaves its level.
• Placeholder masks: assets/voidworks/textures/void_taking/stage_1..12.png, generated by taking_masks() in scripts/build-textures.py. A mask paints, at ≥50% alpha, every pixel the void has reached by its stage. The final art is Draw the final displacement stage masks #107.

For the reviewer

• The masks are drawn as geometry, not by depth stamping as the brief and Just Dire Things do: each face is drawn only over the runs of pixels the void has not reached. 26.1's render pipelines would need a custom pipeline to stamp depth; this keeps the vanilla moving-block and end-portal render types, and looks the same.
• VoidTakesBlockEvent is new public API, the Module's first event, and the seam the game tests watch, as a FakePlayer drops the payload. It has a changelog line.
• A multi-element model, such as the Displacer's own casing, gets the whole mask over each quad, so its blotches don't line up across elements. Left as is.
• Merged main (feat: a void displacer buys zone upgrades with motes #105): the only conflict was DisplacerBlockEntity's imports.

Testing

• sh ./gradlew build: passes, with 22 new JUnit tests (TakingStagesTest, TakingMaskTest, and ShippedMasksTest, which checks the shipped PNGs are 16×16, cumulative, and full by stage 12).
• runGameTestServer: all 257 pass, among them 6 new void_takes_* tests (a Displacer's terrain, a spent Displacer, a Siphon's terrain, lava and air, and nothing for a refused harvest), each seen failing before the wiring.
• /code-review (standards and spec) found no hard violation and no missing requirement; its fixes are in e10e676.
• Not yet checked in the dev client: how it looks. Watch a Displacer's wave, a Siphon harvest of terrain, lava in the Nether and air in the End, and a spent Displacer, and look for z-fighting from about 30 blocks.

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