Production-readiness suite fixes

Surface hydrology policy split, native terrain height cache, watched-edit
hotload cancellation, world-save requestSave with tracked ownership flush,
background-task drain that follows saves admitted mid-drain, peaceful spawn
guard, cached-completion terminal state, IO drains off the owner thread.

Studio pregen at scale: detached historical runtimes keep the configured
noise cache instead of the Studio-wide cache; retired runtimes are no longer
pinned by the volumetric fragment filter's per-thread scratch; pregen mantle
cleanup and native region flushes run on per-run threads instead of the
server thread and the shared Iris IO pool (the flush on the IO pool starved
tectonic plate loads and wedged every Paper worker in Mantle.getChunks);
background-task cancellation claims tasks atomically.

probe/src/main/java/art/arcane/iris/probe/HydrologyPackProbe.java

@@ -2996,7 +2996,7 @@ public final class HydrologyPackProbe {
                 throw new IllegalStateException("Dimension '" + configuration.dimensionKey()
                         + "' has no active hydrology runtime.");
             }
-            validateRequiredProfiles(runtime, configuration.requiredCoverage());
+            validateRequiredProfiles(runtime.settings(), runtime.profileKeys(), configuration.requiredCoverage());
             requiredConfiguredCoverage.addAll(configuredCoverage(runtime));
             validateTileBlockBounds(runtime, configuration);
             for (long tileZ = configuration.minimumTileZ(); tileZ <= configuration.maximumTileZ(); tileZ++) {
@@ -3285,22 +3285,32 @@ public final class HydrologyPackProbe {
         };
     }
 
-    private static void validateRequiredProfiles(
-            IrisHydrologyRuntime runtime,
+    static void validateRequiredProfiles(
+            HydrologyPlannerSettings settings,
+            Set<String> riverProfiles,
             List<CoverageSelector> selectors
     ) {
-        Set<String> riverProfiles = runtime.profileKeys();
         LinkedHashSet<String> deepProfiles = new LinkedHashSet<>();
-        for (HydrologyPlannerSettings.DeepFluid deepFluid : runtime.settings().deepFluids()) {
+        for (HydrologyPlannerSettings.DeepFluid deepFluid : settings.deepFluids()) {
             if (deepFluid.enabled()) {
                 deepProfiles.add(deepFluid.id());
             }
         }
+        LinkedHashSet<String> poolProfiles = new LinkedHashSet<>();
+        for (HydrologyPlannerSettings.SurfacePool pool : settings.surfacePools()) {
+            if (pool.enabled()) {
+                poolProfiles.add(pool.id());
+            }
+        }
         for (CoverageSelector selector : selectors) {
             if (selector.profileKey().equals("*")) {
                 continue;
             }
-            Set<String> configuredProfiles = selector.type().isDeepFluid() ? deepProfiles : riverProfiles;
+            Set<String> configuredProfiles = switch (selector.type()) {
+                case STANDING_POOL -> poolProfiles;
+                case DEEP_POOL, DEEP_CHANNEL -> deepProfiles;
+                default -> riverProfiles;
+            };
             if (!configuredProfiles.contains(selector.profileKey())) {
                 throw new IllegalArgumentException("Required coverage selector " + selector.label()
                         + " references a profile not configured for that feature family.");

probe/src/main/java/art/arcane/iris/probe/HydrologyPlannerProbe.java

@@ -1,5 +1,7 @@
 package art.arcane.iris.probe;
 
+import art.arcane.iris.engine.hydrology.policy.SurfaceRiverPolicy;
+
 import art.arcane.iris.engine.hydrology.HydrologyColumnSample;
 import art.arcane.iris.engine.hydrology.HydrologyFeatureRef;
 import art.arcane.iris.engine.hydrology.HydrologyFeatureType;
@@ -272,7 +274,8 @@ public final class HydrologyPlannerProbe {
                 HydrologyPlannerSettings.Geometry.defaults(),
                 List.of(), List.of(),
                 0D,
-                HydrologyPlannerSettings.SeaCaves.disabled()
+                HydrologyPlannerSettings.SeaCaves.disabled(),
+                HydrologyPlannerSettings.SurfacePolicyBounds.NONE
         );
     }
 
@@ -314,7 +317,9 @@ public final class HydrologyPlannerProbe {
                     List.of("water"), List.of()
             ,
                 Double.NaN,
-                null, Double.NaN, true);
+                null, Double.NaN, true,
+                SurfaceRiverPolicy.INHERIT
+        );
         };
     }
 
@@ -367,7 +372,9 @@ public final class HydrologyPlannerProbe {
                     List.of("water"), List.of()
             ,
                 Double.NaN,
-                null, Double.NaN, true);
+                null, Double.NaN, true,
+                SurfaceRiverPolicy.INHERIT
+        );
         };
     }
 
@@ -493,7 +500,8 @@ public final class HydrologyPlannerProbe {
                 HydrologyPlannerSettings.Geometry.defaults(),
                 List.of(deepFluid), List.of(),
                 0D,
-                HydrologyPlannerSettings.SeaCaves.disabled()
+                HydrologyPlannerSettings.SeaCaves.disabled(),
+                HydrologyPlannerSettings.SurfacePolicyBounds.NONE
         );
     }
 
@@ -542,7 +550,8 @@ public final class HydrologyPlannerProbe {
                     Double.NaN,
                     null,
                     Double.NaN,
-                    true
+                    true,
+                    SurfaceRiverPolicy.INHERIT
             );
         };
     }
@@ -578,7 +587,9 @@ public final class HydrologyPlannerProbe {
                 List.of("water"), List.of()
         ,
                 Double.NaN,
-                null, Double.NaN, true);
+                null, Double.NaN, true,
+                SurfaceRiverPolicy.INHERIT
+        );
     }
 
     private static long[] parseSeeds(String[] arguments) {