Add IGlyphRasterizer and GlyphBitmap for font-agnostic glyph rasterization

Defines a public interface and data type so rendering consumers can
provide their own image library implementation (e.g. Magick.NET Q8 vs
Q16-HDRI) without coupling the renderer to a specific flavor.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: sebgod

src/DIR.Lib/IGlyphRasterizer.cs

@@ -0,0 +1,15 @@
+namespace DIR.Lib;
+
+/// <summary>
+/// Rasterizes individual glyphs into RGBA pixel data.
+/// Consumers provide an implementation backed by their preferred image library.
+/// </summary>
+public interface IGlyphRasterizer
+{
+    GlyphBitmap RasterizeGlyph(string fontPath, float fontSize, char character);
+}
+
+/// <summary>
+/// Raw RGBA bitmap of a single rasterized glyph.
+/// </summary>
+public readonly record struct GlyphBitmap(byte[] Rgba, int Width, int Height);