Overhaul Nether atmosphere and portal corruption

[xefensor]

Summary

• Restore one classic dark-red fog treatment and a darker ambient-light floor across the Nether.
• Make Crimson and Warped Forest vegetation sparse and desert-like while preserving terrain, resources, structures, and mobs.
• Prevent Dried Ghasts from natural Nether-fossil generation without removing the block or Happy Ghast progression.
• Add permanent deterministic Overworld portal drain with death and corruption fronts, portal-area radius scaling, an explicit material palette, protection hooks, bounded travel pulses, and linear lava-source resistance.

Player or developer impact

Players get a darker, visually unified Nether and portals that gradually drain nearby Overworld terrain. Larger portals affect larger areas. Lava sources inside the full affected sphere linearly slow both fronts and stop them when their count matches the portal interior area. Flowing lava does not count and lava is not consumed.

Pack authors can extend the semantic portal-drain tags. Claim integrations can deny individual edits through NETHER_PORTAL_DRAIN. The implementation scans only loaded chunks, performs bounded work, and never force-loads terrain.

Validation

• ./gradlew build
• ./gradlew runGameTestServer, when relevant
• Focused in-game testing, when relevant
• Dedicated-server or multiplayer testing, when relevant

Passed exact selectors:

• retold:nether_forests_use_sparse_desert_vegetation
• retold:nether_fossils_omit_dried_ghasts
• retold:nether_portal_drain_maps_materials_and_protects_valuable_blocks
• retold:nether_portal_drain_uses_explicit_palette
• retold:nether_portal_drain_advances_outward
• retold:nether_portal_drain_scales_with_portal_area
• retold:nether_portal_travel_pulses_are_bounded
• retold:nether_portal_lava_sources_linearly_resist_spread

The final ./gradlew build passed compilation, JUnit, PMD, checks, and assembly. git diff --check also passed.

Not performed: subjective visual approval, natural multi-seed worldgen inspection, long-duration portal pacing, existing-world checks, multiplayer, or dedicated-server testing. These remain documented verification risks.

AI assistance

• I reviewed and understand all submitted changes, including AI-assisted output.

OpenAI Codex materially assisted design iteration, implementation, documentation, and focused automated validation. The developer selected the player-facing behavior and exact portal palette. Automated build and focused GameTests verified the implementation; subjective visual testing remains with the developer.

Documentation and release notes

• Updated CHANGELOG.md for player-facing or notable technical changes
• Updated README or system documentation when behavior or architecture changed
• Added or updated tests where practical

Visual evidence

No screenshots or video were captured. Per the repository agent policy and developer instruction, the developer owns subjective in-game visual testing and final approval.

Licensing

• I have the right to contribute every included file.
• Code and non-asset contributions are provided under LICENSE-CODE.md.
• No creative assets are included in this pull request.