Merge master into 3D terrain

- Resolve IrisTerrainNormalActuator by keeping master's primitive riverWaterSurfaceStream lookup and primarySurfaceLayerOrNull accessors while retaining the terrain column, span, layer surface and ceiling locals that write air through terrain3D gaps
- Resolve HydrologyFootprintCompiler by keeping the MaterializedSurface planned-surface record with terrain ownership beside master's package-private SurfaceRasterIndex, which retains its ownsTerrain override
- Resolve HydrologyCaveContainmentPlanner by dropping the nested RecordingCaveVoxelView, SurfaceObservations and ViewObservations types that master promoted to their own files
- Port the surface-observation ownership check into CaveSurfaceObservations so cached cave validations replay both height and ownership observations through Observation.matches
- Resolve PackExportClosureTest by keeping master's retirement of the source-text packager guard while retaining the snippet export tests, and restore the Files and Path imports those tests use
- Resolve IrisBiomeLayerSlopeSourceTest by folding the lower-ledge slope palette case into master's lockLayers parameterization
- Recapture the studio schema digest for the terrain3D biome property, its IrisTerrain3D definition, the terrain-3d snippet enumeration and the reworded upperDimensionCarving description, verified by reconstructing the previous digest from the stripped schema

probe/build.gradle

@@ -21,6 +21,7 @@ dependencies {
     implementation(project(':core'))
     compileOnly(libs.spigot)
     testImplementation('junit:junit:4.13.2')
+    testImplementation(testFixtures(project(':core')))
     testImplementation(volmLibCoordinate) {
         transitive = false
     }