feat(D.2b): textured-sprite path in TextRenderer + UV-rect DrawSprite

Add uUseTexture==2 (RGBA modulate) branch to ui_text.frag so dat sprites
can be drawn through the existing 2D batcher without touching the font path.

TextRenderer gains _spriteBufs (per-GL-handle List<float>), DrawSprite(), and
a Flush block that issues one draw call per distinct texture with uUseTexture=2.
Also adds DepthMask(false) in the state-save block (restored to true after) to
prevent the transparent-quad pass from writing depth and corrupting the 3D scene
if the UI is flushed mid-frame.

TextureCache gains GetOrUpload(surfaceId, out width, out height) — caches pixel
dimensions alongside the GL handle so UI 9-slice geometry can compute slice UVs
from the source image size without a second decode.

UiRenderContext gains a DrawSprite forwarder that applies the current 2D
translate stack, matching the DrawRect / DrawRectOutline pattern.

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

src/AcDream.App/Rendering/Shaders/ui_text.frag

@@ -7,10 +7,13 @@ uniform sampler2D uTex;
 uniform int uUseTexture;
 
 void main() {
-    if (uUseTexture != 0) {
+    if (uUseTexture == 1) {
         // Font atlas is a single-channel R8 texture; red = coverage alpha.
         float coverage = texture(uTex, vUv).r;
         FragColor = vec4(vColor.rgb, vColor.a * coverage);
+    } else if (uUseTexture == 2) {
+        // RGBA dat sprite (decoded to RGBA8); modulate by tint/alpha.
+        FragColor = texture(uTex, vUv) * vColor;
     } else {
         FragColor = vColor;
     }

src/AcDream.App/Rendering/TextRenderer.cs

@@ -29,6 +29,7 @@ public sealed unsafe class TextRenderer : IDisposable
 
     private readonly List<float> _textBuf = new(8192);
     private readonly List<float> _rectBuf = new(1024);
+    private readonly Dictionary<uint, List<float>> _spriteBufs = new();
     private int _textVerts;
     private int _rectVerts;
     private Vector2 _screenSize;
@@ -64,6 +65,7 @@ public sealed unsafe class TextRenderer : IDisposable
         _screenSize = screenSize;
         _textBuf.Clear();
         _rectBuf.Clear();
+        foreach (var b in _spriteBufs.Values) b.Clear();
         _textVerts = 0;
         _rectVerts = 0;
     }
@@ -129,6 +131,22 @@ public sealed unsafe class TextRenderer : IDisposable
         }
     }
 
+    /// <summary>
+    /// Draw a textured sprite quad in screen pixel space with an explicit
+    /// source-UV rectangle (for 9-slice / atlas sub-regions). Batched per
+    /// GL texture handle; flushed with uUseTexture=2 (RGBA modulate).
+    /// </summary>
+    public void DrawSprite(uint texture, float x, float y, float w, float h,
+        float u0, float v0, float u1, float v1, Vector4 tint)
+    {
+        if (!_spriteBufs.TryGetValue(texture, out var buf))
+        {
+            buf = new List<float>(256);
+            _spriteBufs[texture] = buf;
+        }
+        AppendQuad(buf, x, y, w, h, u0, v0, u1, v1, tint);
+    }
+
     private static void AppendQuad(List<float> buf,
         float x, float y, float w, float h,
         float u0, float v0, float u1, float v1, Vector4 color)
@@ -159,7 +177,9 @@ public sealed unsafe class TextRenderer : IDisposable
     /// <summary>Upload + draw accumulated rects + text. font may be null if only DrawRect was used.</summary>
     public void Flush(BitmapFont? font)
     {
-        if (_textVerts == 0 && _rectVerts == 0) return;
+        bool hasSprites = false;
+        foreach (var b in _spriteBufs.Values) if (b.Count > 0) { hasSprites = true; break; }
+        if (_textVerts == 0 && _rectVerts == 0 && !hasSprites) return;
 
         _shader.Use();
         _shader.SetVec2("uScreenSize", _screenSize);
@@ -173,6 +193,7 @@ public sealed unsafe class TextRenderer : IDisposable
         bool wasCull  = _gl.IsEnabled(EnableCap.CullFace);
         _gl.Disable(EnableCap.DepthTest);
         _gl.Disable(EnableCap.CullFace);
+        _gl.DepthMask(false);
         _gl.Enable(EnableCap.Blend);
         _gl.BlendFunc(BlendingFactor.SrcAlpha, BlendingFactor.OneMinusSrcAlpha);
 
@@ -195,7 +216,23 @@ public sealed unsafe class TextRenderer : IDisposable
             _gl.DrawArrays(PrimitiveType.Triangles, 0, (uint)_textVerts);
         }
 
+        // RGBA dat sprites — one draw call per distinct GL texture.
+        if (hasSprites)
+        {
+            _shader.SetInt("uUseTexture", 2);
+            _gl.ActiveTexture(TextureUnit.Texture0);
+            _shader.SetInt("uTex", 0);
+            foreach (var kv in _spriteBufs)
+            {
+                if (kv.Value.Count == 0) continue;
+                _gl.BindTexture(TextureTarget.Texture2D, kv.Key);
+                UploadBuffer(kv.Value);
+                _gl.DrawArrays(PrimitiveType.Triangles, 0, (uint)(kv.Value.Count / FloatsPerVertex));
+            }
+        }
+
         // Restore GL state.
+        _gl.DepthMask(true);
         if (!wasBlend) _gl.Disable(EnableCap.Blend);
         if (wasCull)   _gl.Enable(EnableCap.CullFace);
         if (wasDepth)  _gl.Enable(EnableCap.DepthTest);

src/AcDream.App/Rendering/TextureCache.cs

@@ -14,6 +14,7 @@ public sealed unsafe class TextureCache : Wb.ITextureCachePerInstance, IDisposab
     private readonly GL _gl;
     private readonly DatCollection _dats;
     private readonly Dictionary<uint, uint> _handlesBySurfaceId = new();
+    private readonly Dictionary<uint, (int w, int h)> _sizeBySurfaceId = new();
     /// <summary>
     /// Composite cache for surface-with-override-origtex entries (Phase 5
     /// TextureChanges). Key = (baseSurfaceId, overrideOrigTextureId),
@@ -80,6 +81,28 @@ public sealed unsafe class TextureCache : Wb.ITextureCachePerInstance, IDisposab
         return h;
     }
 
+    /// <summary>
+    /// Like <see cref="GetOrUpload(uint)"/> but also returns the decoded
+    /// pixel dimensions. UI 9-slice geometry needs the source size to
+    /// compute slice UVs. Cached alongside the handle.
+    /// </summary>
+    public uint GetOrUpload(uint surfaceId, out int width, out int height)
+    {
+        if (_handlesBySurfaceId.TryGetValue(surfaceId, out var existing)
+            && _sizeBySurfaceId.TryGetValue(surfaceId, out var sz))
+        {
+            width = sz.w; height = sz.h;
+            return existing;
+        }
+
+        var decoded = DecodeFromDats(surfaceId, origTextureOverride: null, paletteOverride: null);
+        uint h = UploadRgba8(decoded);
+        _handlesBySurfaceId[surfaceId] = h;
+        _sizeBySurfaceId[surfaceId] = (decoded.Width, decoded.Height);
+        width = decoded.Width; height = decoded.Height;
+        return h;
+    }
+
     /// <summary>
     /// Alpha-channel histogram for one decoded texture. Used to diagnose
     /// "why are clouds not transparent" — if cloud textures come out with

src/AcDream.App/UI/UiRenderContext.cs

@@ -53,6 +53,11 @@ public sealed class UiRenderContext
     public void DrawRectOutline(float x, float y, float w, float h, Vector4 color, float thickness = 1f)
         => TextRenderer.DrawRectOutline(_current.X + x, _current.Y + y, w, h, color, thickness);
 
+    public void DrawSprite(uint texture, float x, float y, float w, float h,
+        float u0, float v0, float u1, float v1, Vector4 tint)
+        => TextRenderer.DrawSprite(texture,
+            _current.X + x, _current.Y + y, w, h, u0, v0, u1, v1, tint);
+
     public void DrawString(string text, float x, float y, Vector4 color, BitmapFont? font = null)
     {
         var f = font ?? DefaultFont;