feat: support VirtualEntities across Spigot, Paper, and Folia

[twme-ai]

Summary

• replace EntityLib with VirtualEntities v0.9.0 for freeze-mode display entities
• add Paper-first scheduler routing and thread-safe shared state for Folia
• support Spigot, Paper, and Folia from 1.19.4 through 26.2 while retaining MiniMessage
• require JDK 25 for builds and emit Java 17-compatible bytecode for legacy servers
• add Mineflayer integration coverage and endpoint runtime jobs to CI

Verification

• mvn -B clean package: 8 tests passed with JDK 25
• Spigot 1.19.4 + Java 17: Mineflayer command, item, display spawn, and removal checks passed
• Paper 1.19.4 + Java 17: Mineflayer checks passed
• Folia 1.19.4 + Java 17: Mineflayer checks passed without region-thread violations
• Spigot, Paper, and Folia 26.2 + Java 25: server startup and plugin enable checks passed

Mineflayer 4.37.1 cannot load the 26.2 protocol yet, so the 26.2 CI cases verify real server startup and clean plugin enablement; the full Mineflayer flow runs on the lower endpoint.