Ice Caves Backport 0.1.5

Sorry about the last patch making Ice Caves generate as buggy, one-chunk snippets. This update fixes that generation regression. Thanks for testing and reporting it!

• Fix Ice Caves being cut into isolated chunk-sized patches, with continuous cave coating and smoother biome transitions across chunk borders.

• Fix unnatural water generation underneath brittle ice, keeping crevasse water contained and level.

• Replace guaranteed-floor crystal placement with the snapshot's random-height, downward-scan placement sequence. Restore failed attempts instead of covering small cave floors.

• Restore the snapshot's snow-layer placement sequence instead of painting a percentage of every floor.

• Backport the triangular offset sampler to pre-26 versions and adapt the JSON encoding for 1.20.1 without changing its sampling rules.

• Register snow, crystals and crevasses only in Ice Caves, in native order. Use minecraft:ice_crystals, minecraft:snow_layers, minecraft:ice_crevasse and the minecraft:crevasse feature type.

• Correct the reversed melting-tip obstruction check: a heated hanging tip falls into free space, as in the snapshot.

• Retain native crystal light level 4 and the icicle melting threshold above 4; crystal lights do not add together to exceed it.

• Regression servers stop immediately on startup/test failure and reject overlapping runs of the same suite, in addition to the existing five-minute watchdog.

Generation changes affect new chunks only. Existing decorated chunks are not rewritten. The surface-rule compatibility helper also uses minecraft:ice_cave, as requested. It is a backport implementation detail rather than a native snapshot feature.