Port IronAgeFurniture to NeoForge 1.21.1

[SkyBlade1978]

Summary

• port IronAgeFurniture to Minecraft 1.21.1 and NeoForge 21.1.247 on Java 21
• retain optional Biomes O' Plenty, Oh The Biomes We've Gone, and Immersive Engineering furniture with native NeoForge conditions and no hard dependencies
• carry forward recipe advancement, flammability, lava-lamp, texture, and compatibility fixes
• preserve waterlogging both when furniture is placed underwater and after falling into water
• provide reproducible Eclipse/Buildship setup and release documentation for 0.3.0.121012

Verification

• user-completed in-game acceptance, including underwater placement and falling-lamp waterlogging
• gradlew.bat clean check build javadoc prepareEclipse verifyEclipseProductionClasspath verifyReleaseArtifacts verifyReleaseChecksums --no-daemon
• second clean build reproduced all three release artifact SHA-256 hashes
• main jar: C6A7908EC60AA2488B918C5BF66610FE7967DB13546DAC47132FBC3CC33F4433
• validated 15,348 resource JSON documents, 7,206 locale keys, 1,536 conditional recipes, and 1,520 conditional advancements