fix(render) #443: private viewports take the ring transform path

The paperdoll was visible only in portal space. Root cause: the classic
WbDrawDispatcher.Draw path appended its transforms into the SHARED world
transform frame (WorldTransformFrameArena.Append) with a non-zero base
instance, but the default mesh shaders index every parallel per-instance
array - clip slots, light sets, indoor, OPACITY, selection lighting,
detail category - zero-based; only the packed world submission's shader
convention subtracts the shared-arena prefix. With a world frame active
the doll drew all instances at per-instance opacity 0 into a cleared
target: counted draws, blank pixels, deterministic. Portal space worked
because no world transform frame is active there, so the same code took
the ring path with base 0. The private viewports are the only production
consumers of the classic path, hiding the defect everywhere else.

Fix: WbDrawDispatcher.NextClassicDrawIsPrivatePass - the private
viewport renderer marks its draw and WriteWorldTransformSection routes
private passes onto the plain ring path unconditionally (self-contained
render state: the private pass owns its own camera, lighting, and
target, and must not depend on the world frame's pose address space).

Also landed, each independently justified:
- Per-GPU-flight-slot private targets (PrivateViewportFlightTargets),
  restoring the pre-f6fe0f2a design: that revert's claim that frame
  submission order protects the single target's write->sample transition
  is not guaranteed across Vulkan command buffers. Per-slot completed
  scenes fix the cleared-sibling-after-reveal wart the old attempt had.
- Paperdoll resource preparation moved to the frame resource phase
  (IPrivateEntityViewportResourcePreparation) before world draws consume
  the bounded composite-upload budget.
- The presenter redresses on every dirty edge (an appearance-equal clone
  can pin retired readiness across generations; the renderer's two-phase
  promote keeps the last completed image visible during replacement),
  publishes only non-zero handles, and clears the viewport exactly once
  at the explicit character-session boundary.

Verified live on the clean build: doll visible in the NORMAL world,
visible through portal space, and still visible after arrival - the
exact reported repro cycle. 26 paperdoll/private-viewport/preparation
tests plus 60 renderer-suite tests pass; owner visual gate pending.

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

src/AcDream.App/Rendering/Wb/WbDrawDispatcher.Rhi.cs

@@ -924,6 +924,24 @@ public sealed unsafe partial class WbDrawDispatcher
         return new RhiSection(allocation.Buffer, allocation.OffsetBytes, (uint)byteCount);
     }
 
+    /// <summary>
+    /// #443 — the next classic draw is a PRIVATE pass (paperdoll, appraisal,
+    /// chargen preview) and must take the plain ring transform path with
+    /// <c>firstInstance = 0</c>, never an append into the shared world
+    /// transform frame. The default mesh shaders index every parallel
+    /// per-instance array (clip slots, light sets, indoor, opacity, selection,
+    /// detail category) zero-based — only the packed world submission's
+    /// shader convention subtracts a shared-arena prefix — so an
+    /// arena-appended classic draw with a non-zero base reads zeroed
+    /// per-instance data (opacity 0 ⇒ an invisible doll whenever a world
+    /// frame is active; portal space worked only because the arena was
+    /// inactive there). The private pass owns its own camera, lighting, and
+    /// target; per the self-contained-render-state rule it must not depend on
+    /// the world frame's pose address space at all. Consumed and cleared by
+    /// the next <see cref="WriteWorldTransformSection"/>.
+    /// </summary>
+    internal bool NextClassicDrawIsPrivatePass;
+
     private RhiSection WriteWorldTransformSection(
         IGpuFrame frame,
         ReadOnlySpan<float> matrixFloats,
@@ -939,7 +957,9 @@ public sealed unsafe partial class WbDrawDispatcher
         ObserveOrdinaryTransformDemand(
             frame.Serial,
             checked((uint)(matrixFloats.Length / 16)));
-        if (!_worldTransformFrames.IsActive)
+        bool privatePass = NextClassicDrawIsPrivatePass;
+        NextClassicDrawIsPrivatePass = false;
+        if (privatePass || !_worldTransformFrames.IsActive)
         {
             firstInstance = 0;
             return WriteRingSection(frame, matrixFloats);