fix #429: allocation-free shadow topology rebuild + churn-frame pipelining

The directional-shadow topology rebuilt on every streaming-churn frame
and was the measured body of the run-hitch stalls (701 of 708 baseline
stalls alloc-correlated):

- The draw sort comparer's enum-vs-enum CompareTo bound to
  Enum.CompareTo(object) and boxed BOTH operands on every comparison —
  a constant ~38.9 MB of garbage per topology rebuild (~4M boxes),
  handing the GC a forced gen0 collection mid-frame. The full ~100k-draw
  sort is replaced outright: draws hash-group by exact DrawKey in one
  O(n) pass over retained chained-index arrays, and only the
  few-thousand DISTINCT group keys sort (order-preserving packed
  material|cull|firstIndex|baseVertex + count|slot|layer|foliage keys,
  first-appearance tie-break) — bit-identical emission order to the old
  stable sort, near-zero allocation, and no per-draw comparisons at all.
- The caster frame sorts 4-byte indices keyed on SortKey.Value instead
  of shuffling multi-hundred-byte records through a boxing comparer.
- Owner-approved pipelining: on a frame whose shadow inputs just changed
  (the same frame already paying frame-view/landscape rebuilds), the
  caster-frame and prepared-draws topology rebuilds defer to the next
  quieter frame, capped at two consecutive deferrals — inside the GPU
  fence depth, so retained draws never reference a released arena range.
  First build, generation change, caster BuildSequence change, and
  journal overflow force the immediate path; deferred refreshes skip
  identity-mismatched journal rows.

Owner-accepted in both presentation modes: stall frames 5.8/s -> ~0.45/s
uncapped (0.49/s capped), median stall 20.3 -> 13.7 ms, >25 ms frames
near zero, 275 fps uncapped baseline restored. Allocation gate: a warmed
topology rebuild must allocate <2 KiB (DirectionalShadowPreparedDrawTests).
docs/ISSUES.md carries the full evidence trail; the residual
content-proportional rebuild milliseconds are filed as the
incremental-topology successor, and the pre-existing town-view scaling
latch is filed as #432.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/AcDream.App/Rendering/Packs/AtmosphericPostProcessGraph.cs

@@ -197,6 +197,10 @@ internal sealed class AtmosphericPostProcessGraph :
     private readonly bool _fuseLowPostProcess;
     private readonly AtmosphericCpuStageProfiler? _cpuStageProfiler;
     private readonly DirectionalShadowCasterFrame _shadowCasters = new();
+    // #429 owner-approved pipelining state — see RenderDirectionalShadows.
+    private ulong _lastObservedSceneShadowRevision;
+    private long _lastObservedAvailabilityVersion;
+    private int _shadowRebuildDeferrals;
     private TargetSet? _targets;
     private AtmosphericFrameInputs _lastInputs;
     private DirectionalSunShadowDiagnostics _lastShadowDiagnostics;
@@ -395,7 +399,33 @@ internal sealed class AtmosphericPostProcessGraph :
         bool measureCpuStages = _cpuStageProfiler is not null
             && AtmosphericCpuStageProfiler.ShouldMeasure(frame.Serial);
         long stageStarted = measureCpuStages ? Stopwatch.GetTimestamp() : 0L;
-        _shadowCasters.Build(in scene);
+        // #429 owner-approved pipelining: on a frame where the shadow inputs
+        // just CHANGED (streaming publish churn — the same frame already pays
+        // the frame-view and landscape rebuilds), keep the retained shadow
+        // topology and let the rebuild land on the next quieter frame. Capped
+        // at two consecutive deferrals: the GPU frame fence is deeper than
+        // that, so retained prepared draws can never reference an arena range
+        // that was released AND reused while deferred. A caster rebuild the
+        // frame forces anyway (first build, generation change, journal
+        // overflow) re-enables the draws rebuild in the same frame — the
+        // prepared draws must never index a caster frame they were not built
+        // from.
+        ulong sceneShadowRevision = scene.DirectionalShadowTopologyRevision;
+        long availabilityVersion = worldMeshes.DirectionalShadowAvailabilityVersion;
+        bool shadowInputsChanged =
+            sceneShadowRevision != _lastObservedSceneShadowRevision
+            || availabilityVersion != _lastObservedAvailabilityVersion;
+        _lastObservedSceneShadowRevision = sceneShadowRevision;
+        _lastObservedAvailabilityVersion = availabilityVersion;
+        bool allowTopologyRebuild =
+            !shadowInputsChanged || _shadowRebuildDeferrals >= 2;
+        ulong casterSequenceBefore = _shadowCasters.BuildSequence;
+        _shadowCasters.Build(in scene, allowTopologyRebuild);
+        if (_shadowCasters.BuildSequence != casterSequenceBefore)
+            allowTopologyRebuild = true;
+        _shadowRebuildDeferrals = allowTopologyRebuild
+            ? 0
+            : _shadowRebuildDeferrals + 1;
         long casterBuildFinished = measureCpuStages ? Stopwatch.GetTimestamp() : 0L;
         AuthoredCelestialShadowSource source = world.CelestialShadowSource;
         var environment = new DirectionalShadowEnvironmentInput(
@@ -433,7 +463,8 @@ internal sealed class AtmosphericPostProcessGraph :
                 Preset.Semantic,
                 frame.Serial),
             MeasureCpuStages: measureCpuStages,
-            AtmosphericFrame: shadowAtmosphericFrame);
+            AtmosphericFrame: shadowAtmosphericFrame,
+            AllowTopologyRebuild: allowTopologyRebuild);
         long environmentFinished = measureCpuStages ? Stopwatch.GetTimestamp() : 0L;
         _lastShadowCasterCount = _shadowCasters.Stats.Accepted;
         _lastShadowClassificationCalls = _shadowCasters.Stats.TopologyRebuilt ? 1 : 0;