fix(agc): merge Prospero attrib-table formats onto IR vertex inputs (#556)

IR-discovered BufferLoadFormat often keeps a stale float sharp format;
patch DataFormat/offset from the AGC attrib table (semantic index),
allow offen fetches, and map quirks 113/121 through NarrowVk for host
vertex input.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/SharpEmu.Libs/Gpu/Metal/MetalVideoPresenter.Draws.cs

@@ -1225,8 +1225,11 @@ internal static partial class MetalVideoPresenter
                 ? vertexBuffer.Stride
                 : Math.Max(vertexBuffer.ComponentCount, 1) * 4;
             MetalNative.Send(layout, MetalNative.Selector("setStride:"), (nint)stride);
-            // MTLVertexStepFunction.PerVertex = 1.
-            MetalNative.Send(layout, MetalNative.Selector("setStepFunction:"), 1);
+            // MTLVertexStepFunction: PerVertex = 1, PerInstance = 2.
+            MetalNative.Send(
+                layout,
+                MetalNative.Selector("setStepFunction:"),
+                vertexBuffer.PerInstance ? 2 : 1);
         }
 
         return descriptor;