[adamico]

Summary

Flowing water and lava washed a pipe away and dropped it, as they would a torch: vanilla's FlowingFluid.canHoldAnyFluid replaces any block that doesn't blocksMotion(), and a pipe's thin core-and-arms shape isn't solid by calculateSolid's rule. The pipe and the creative pipe now force solid, so a fluid stops at them as at a wall. No new blockstate, no waterlogging (turned down in triage: a waterlogged pipe would read as a water source to the Pump).

 PIPE / CREATIVE_PIPE properties
-  strength(...).sound(COPPER).requiresCorrectToolForDrops().noOcclusion()
+  strength(...).sound(COPPER).requiresCorrectToolForDrops().noOcclusion().forceSolidOn()

Vanilla side effects of a solid pipe (every isSolid()/blocksMotion() caller checked):

• a sign or banner stands on a pipe; a painting or item frame hangs on one
• farmland or a dirt path under a pipe turns to dirt; a cactus beside one breaks
• rain stops at a pipe (the MOTION_BLOCKING heightmap)
• a fluid no longer falls through a pipe as through a hole: it spreads over it
• unchanged: mob pathing (isPathfindable reads the full collision shape), suffocation, and the Outlet's placement rule (ADR 0007), since a pipe still can't be replaced

Evidence

New game tests, fluid_flow_*: a pipe and a creative pipe each beside a water and a lava source, and a pipe under falling water. Each first checks the fluid did spread on an open side, then that the pipe is still there, then takes the source away so the flow stays on the platform.

• Before: fluid_flow_*: 5 failed, e.g. the flowing minecraft:water washed the pipe away (relative: 3, 1, 4)
  After: fluid_flow_*: 5 of 5 runs passed; sh ./gradlew build passes; runGameTestServer: all 136 required tests passed

Merge Danger

Door: two-way

Removing forceSolidOn() puts it back; no saved data or blockstate changes.

Blast Radius: small

Placed pipes in existing worlds become solid on load: farmland or a dirt path under a pipe will turn to dirt on its next update, and a cactus beside a pipe breaks.

Closes #34

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