Port Sky's Grass Slabs to Minecraft 26.3 on NeoForge

[SkyBlade1978]

Summary

• port Sky's Grass Slabs to Minecraft 26.3 and NeoForge 26.3.0.1-beta
• preserve existing block, item, recipe, configuration and saved-world identities
• adapt shovel flattening, crafting, world generation, resources and legacy migration to the 26.3 APIs
• update CI, Eclipse metadata and player-facing documentation for the new target

Verification

• 47 focused tests passed
• all 18 required GameTests passed
• clean build, Javadocs, Eclipse isolation and release audit passed
• fresh and reload dedicated-server starts passed
• two independent builds produced identical main, sources and Javadocs artifacts
• main jar SHA-256: C048A2B1F159A515E136F3665AF500872573B72943C683665324C3A5A84C0C9B
• Java class major 69 and repository hygiene checks passed

Acceptance

Manual client testing passed before this PR was opened.