Support Minecraft 26.3

26.3 changed the chunk generator, biome source and structure placement
APIs, so it gets its own v26_3_R1 binding, compiled against the 26.3
dev bundle. v26_1_R1 keeps covering 26.1.2 and 26.2.

Binding changes against v26_1_R1:
- noise, surface and carvers run through a single buildTerrain stage
- the biome source hands out itself as the BiomeResolver
- structures are generated with a climate sampler, and starts are
  looked up per chunk instead of per section
- structure set salt and frequency come from the spreading placement
- biome colors are read from Vector3f attributes

Data pack format 121 with DataFixerV2603:
- biome mob spawns and creature spawn probability move into the
  natural_mob_spawns and creature_world_gen_spawn_probability attributes
  (26.3 silently ignores the old fields)
- the bed rule's "explodes" becomes "destroy_on_use", and straw beds get
  the same rule, matching vanilla

Model used: Claude Opus 5.5

settings.gradle.kts

@@ -23,6 +23,7 @@ rootProject.name = "Iris"
 
 include(":core", ":core:agent")
 include(
+    ":nms:v26_3_R1",
     ":nms:v26_1_R1",
     ":nms:v1_21_R7",
     ":nms:v1_21_R6",