feat(D.2b): Slice 2 — PaperdollViewportRenderer RTT pass (static doll)

The C# analog of CreatureMode::Render: draws one re-dressed player clone
into a private FBO (RGBA8 + depth24-stencil8) with the fixed DollCamera +
one distant light (retail 0.3,1.9,0.65 @ 2.0), sealed in a GLStateScope so
it can't disturb world/UI GL state. frustum:null ⇒ the doll's synthetic
landblock is always visible ⇒ walked from entry.Entities and drawn from its
current MeshRefs (static pose; idle animation is a later slice).

Manages the FBO directly via GL (ManagedGLFramebuffer is unused/bitrotted
WB infra). UiViewport blit flips V (FBO bottom-left origin vs UI top-left)
so the doll isn't upside-down. Not yet wired — GameWindow hook is next.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/AcDream.App/UI/UiViewport.cs

@@ -19,7 +19,9 @@ public sealed class UiViewport : UiElement
     {
         if (!Visible || TextureHandle == 0) return;
         // Local origin is already at this widget's Left/Top (PushTransform applied by DrawSelfAndChildren).
-        // Draw the full-texture sprite with UV 0..1 and white tint (no color modulation).
-        ctx.DrawSprite(TextureHandle, 0f, 0f, Width, Height, 0f, 0f, 1f, 1f, Vector4.One);
+        // V is FLIPPED (v0=1, v1=0): TextureHandle is an off-screen FBO color texture, whose origin is
+        // bottom-left (GL), while the UI sprite convention is top-left. Without the flip the doll renders
+        // upside-down. (If the doll appears upside-down at the visual gate, this is the line to revisit.)
+        ctx.DrawSprite(TextureHandle, 0f, 0f, Width, Height, 0f, 1f, 1f, 0f, Vector4.One);
     }
 }