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/DirectionalSunShadowRenderer.cs

@@ -38,7 +38,12 @@ internal readonly record struct DirectionalSunShadowRenderInput(
     float ResidentMaximumReachMeters = float.PositiveInfinity,
     bool MeasureGpuTimers = true,
     bool MeasureCpuStages = false,
-    AtmosphericFrameBufferBinding AtmosphericFrame = default);
+    AtmosphericFrameBufferBinding AtmosphericFrame = default,
+    // #429 owner-approved pipelining: false keeps the retained caster/draw
+    // topology this frame (transform refresh only) so the rebuild lands on a
+    // quieter frame. The prepare seams below re-validate and rebuild anyway
+    // whenever deferral would be unsafe.
+    bool AllowTopologyRebuild = true);
 
 internal readonly record struct DirectionalSunShadowCpuStageTicks(
     long EnvironmentGateTicks,
@@ -394,7 +399,9 @@ internal sealed class DirectionalSunShadowRenderer : IDirectionalShadowReceiverS
 
         long cpuStageStarted = input.MeasureCpuStages ? Stopwatch.GetTimestamp() : 0L;
         DirectionalShadowPreparedDraws worldDraws =
-            world.PrepareDirectionalShadowDraws(input.Casters);
+            world.PrepareDirectionalShadowDraws(
+                input.Casters,
+                input.AllowTopologyRebuild);
         DirectionalShadowTerrainPreparedDraws terrainDraws =
             terrain.PrepareDirectionalShadowDraws();
         DirectionalShadowMeshGeometry? worldGeometry =