feat(gfx): retained DrawLayer list in GfxState (thread-safe)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

engine/Age.Engine.Tests/GfxStateTests.cs

@@ -59,4 +59,30 @@ public class GfxStateTests
         Assert.Equal(-1, g.QuerySlot(0x10));
         Assert.NotEqual(-1, g.QuerySlot(0x11));
     }
+
+    [Fact]
+    public void LayersAppendInOrderAndUpdateInPlace()
+    {
+        var g = new GfxState();
+        g.AddOrUpdateLayer(new DrawLayer(0xA, 4, 0, 0, 800, 600, 0, 0));
+        g.AddOrUpdateLayer(new DrawLayer(0xB, 5, 0, 0, 200, 200, 100, 100));
+        g.AddOrUpdateLayer(new DrawLayer(0xA, 4, 0, 0, 800, 600, 0, 50));   // re-draw A -> update in place
+        var s = g.SnapshotLayers();
+        Assert.Equal(2, s.Count);
+        Assert.Equal(0xA, s[0].Handle);           // order preserved (A still first)
+        Assert.Equal(50, s[0].DstY);              // updated
+        Assert.Equal(0xB, s[1].Handle);
+    }
+
+    [Fact]
+    public void EraseRangeAlsoDropsLayers()
+    {
+        var g = new GfxState();
+        g.AddOrUpdateLayer(new DrawLayer(0x10, 4, 0, 0, 10, 10, 0, 0));
+        g.AddOrUpdateLayer(new DrawLayer(0x20, 5, 0, 0, 10, 10, 0, 0));
+        g.EraseRange(0x10, 1);
+        var s = g.SnapshotLayers();
+        Assert.Single(s);
+        Assert.Equal(0x20, s[0].Handle);
+    }
 }

engine/Age.Engine/Model/GfxState.cs

@@ -1,5 +1,10 @@
 namespace Age.Engine.Model;
 
+/// <summary>A retained composite layer captured when the VM executes draw-texture — the host clears and
+/// re-composites the layer set each frame (replacing the immediate-mode permanent canvas). Keyed by the
+/// object <see cref="Handle"/> so a re-draw updates in place and an erase (op 0x1f7) removes it.</summary>
+public readonly record struct DrawLayer(long Handle, int Slot, int SrcX, int SrcY, int W, int H, int DstX, int DstY);
+
 /// <summary>Host-agnostic model of the AGE native gfx command-buffer (reversed in
 /// docs/engine-re.md, gfx op-contract table). One registry maps an object handle to a GfxObject — the
 /// native ctx+0x408 map that op 0x215 queries and the geometry get/set ops share. Each object carries a
@@ -59,10 +64,31 @@ public sealed class GfxState
     /// NOT a create — objects are created lazily by the geometry SET ops (gfx_object_get_or_create).</summary>
     public void EraseRange(long handle, long count)
     {
-        if (count > 1) for (long i = handle; i < handle + count; i++) Release(i);
-        else Release(handle);
+        if (count > 1) for (long i = handle; i < handle + count; i++) { Release(i); RemoveLayers(i); }
+        else { Release(handle); RemoveLayers(handle); }
     }
 
+    // ---- retained composite layers (mutated on the VM thread; snapshotted on the host render thread) ----
+    private readonly List<DrawLayer> _layers = new();
+    private readonly object _lock = new();
+
+    /// <summary>Record a draw: update the layer with this handle in place (preserving composite order), else
+    /// append. Order = execution order = paint order.</summary>
+    public void AddOrUpdateLayer(DrawLayer l)
+    {
+        lock (_lock)
+        {
+            for (int i = 0; i < _layers.Count; i++)
+                if (_layers[i].Handle == l.Handle) { _layers[i] = l; return; }
+            _layers.Add(l);
+        }
+    }
+
+    public void RemoveLayers(long handle) { lock (_lock) { _layers.RemoveAll(l => l.Handle == handle); } }
+
+    /// <summary>An immutable snapshot of the current layers, in composite order (for the host render loop).</summary>
+    public IReadOnlyList<DrawLayer> SnapshotLayers() { lock (_lock) { return _layers.ToArray(); } }
+
     /// <summary>Pack (alpha, rgb) → 0xAARRGGBB, matching op 0x202/0x203's handler bit-manipulation for the
     /// common (non-negative-sentinel) case. The alpha&lt;0 / color&lt;0 native-fetch path is deferred.</summary>
     public static long PackColor(long alpha, long color)