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/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;