收口字体渲染路径以降低 atlas 体积并直接输出 RGB565

将位图字体从 RGBA8888 atlas 改为 1-bit mask，避免继续为字体存储无效颜色信息。
生成端现在按 row-major、MSB-first 将 8 个像素打包进 1 个 uint8_t；运行时由
draw_text 提供目标颜色，并在 DrawContext 中直接把 bit=1 的像素写入 RGB565 framebuffer。同步补充了回归测试与文档，确保颜色替换、背景填充和裁剪语义保持稳定。

Constraint: 本次只收口字体路径，不改 sprite/tilemap 资源格式
Constraint: 运行时保持纯二值绘制语义，不引入 alpha blending 或抗锯齿
Rejected: 继续兼容 RGBA8888 字体输入 | 会保留双路径复杂度且无实际收益
Confidence: high
Scope-risk: moderate
Reversibility: clean
Directive: 后续字体资源必须继续使用 row-major + MSB-first 的 1-bit mask；修改位序前先同步生成脚本、运行时读取和测试
Tested: cmake --build build-win --config Release
Tested: build-win/Release/render_pipeline_tests.exe
Tested: ctest -C Release --output-on-failure
Tested: font_atlas_mask 字节数校验（2808 == ceil(208*108/8)）
Not-tested: Demo/设备上的人工目视文字显示效果

src/Apps/Demo/main.cpp

@@ -133,11 +133,9 @@ int main(int argc, char* argv[])
 	Platform::SteadyTimeSource time_source;
 
 	RenderData::BitmapFont font;
-	font.atlas = RenderData::Image(
-		font_atlas_pixels,
-		font_atlas_width,
-		font_atlas_height,
-		RenderData::PixelFormat::RGBA8888);
+	font.mask_bits = font_atlas_mask;
+	font.atlas_width = font_atlas_width;
+	font.atlas_height = font_atlas_height;
 	font.char_w = font_char_w;
 	font.char_h = font_char_h;
 	font.columns = font_columns;

src/Core/Draw2D/DrawContext.cpp

@@ -325,31 +325,68 @@ namespace Core
 		}
 	}
 
+	void DrawContext::draw_font_mask_region(const RenderData::BitmapFont& font,
+	                                        int32_t src_x, int32_t src_y,
+	                                        int32_t src_w, int32_t src_h,
+	                                        int32_t dst_x, int32_t dst_y,
+	                                        uint16_t pixel)
+	{
+		Core::FramePixel* dst = static_cast<Core::FramePixel*>(frameBuffer->get_buffer());
+		const int32_t fb_w = frameBuffer->get_width();
+
+		for (int32_t row = 0; row < src_h; ++row)
+		{
+			const int32_t atlas_row = src_y + row;
+			const int32_t dst_y_abs = dst_y + row;
+			Core::FramePixel* dst_row = dst + dst_y_abs * fb_w + dst_x;
+
+			for (int32_t col = 0; col < src_w; ++col)
+			{
+				const int32_t atlas_x = src_x + col;
+				const int32_t bit_index = atlas_row * font.atlas_width + atlas_x;
+				const uint8_t packed = font.mask_bits[bit_index >> 3];
+				const uint8_t bit_mask = static_cast<uint8_t>(0x80u >> (bit_index & 7));
+				if ((packed & bit_mask) == 0u) continue;
+				dst_row[col] = pixel;
+			}
+		}
+	}
+
 	void DrawContext::draw_text(const RenderData::BitmapFont& font, int32_t x, int32_t y,
 	                            const RenderData::Color& color, const char* text)
 	{
-		if (!font.atlas.pixels || !text) return;
+		if (!font.mask_bits || !text) return;
 
-		const uint32_t rgba = color.to_rgba();
 		const int32_t cw = font.char_w;
 		const int32_t ch = font.char_h;
-		const int32_t atlas_w = font.atlas.width;
-		const uint32_t* src = static_cast<const uint32_t*>(font.atlas.pixels);
+		const int32_t atlas_w = font.atlas_width;
+		const int32_t atlas_h = font.atlas_height;
 		const int32_t screen_w = frameBuffer->get_width();
 		const int32_t screen_h = frameBuffer->get_height();
+		if (cw <= 0 || ch <= 0 || font.columns <= 0 || atlas_w <= 0 || atlas_h <= 0) return;
+
+		const Core::FramePixel glyph_pixel = Core::rgba_to_frame_pixel(color.to_rgba());
 
 		int32_t cursor_x = x;
 		for (const char* p = text; *p; ++p)
 		{
 			const int32_t index = static_cast<int32_t>(*p) - font.first_char;
-			if (index < 0) { cursor_x += cw; continue; }
+			if (index < 0)
+			{
+				cursor_x += cw;
+				continue;
+			}
 
 			const int32_t col = index % font.columns;
 			const int32_t row = index / font.columns;
 			const int32_t src_x = col * cw;
 			const int32_t src_y = row * ch;
+			if (src_x < 0 || src_y < 0 || src_x + cw > atlas_w || src_y + ch > atlas_h)
+			{
+				cursor_x += cw;
+				continue;
+			}
 
-			// 整字符在屏幕外则跳过
 			if (cursor_x + cw <= 0 || cursor_x >= screen_w || y + ch <= 0 || y >= screen_h)
 			{
 				cursor_x += cw;
@@ -362,18 +399,14 @@ namespace Core
 			if (y + ch > screen_h) sy_end = screen_h - y;
 			if (cursor_x < 0) sx_start = -cursor_x;
 			if (cursor_x + cw > screen_w) sx_end = screen_w - cursor_x;
-
-			for (int32_t sy = sy_start; sy < sy_end; ++sy)
-			{
-				const uint32_t* atlas_row = src + (src_y + sy) * atlas_w;
-				const int32_t dst_y_abs = y + sy;
-				for (int32_t sx = sx_start; sx < sx_end; ++sx)
-				{
-					const uint32_t pixel = atlas_row[src_x + sx];
-					if ((pixel & 0xFF) == 0) continue;
-					frameBuffer->set_pixel_unsafe(cursor_x + sx, dst_y_abs, rgba);
-				}
-			}
+			draw_font_mask_region(font,
+			                      src_x + sx_start,
+			                      src_y + sy_start,
+			                      sx_end - sx_start,
+			                      sy_end - sy_start,
+			                      cursor_x + sx_start,
+			                      y + sy_start,
+			                      glyph_pixel);
 			cursor_x += cw;
 		}
 	}

src/Core/Draw2D/DrawContext.h

@@ -35,6 +35,12 @@ namespace Core
 		Rasterizer::Rasterizer* rasterizer;
 		Rasterizer::TriangleRasterizer* triangleRasterizer;
 
+		void draw_font_mask_region(const RenderData::BitmapFont& font,
+		                           int32_t src_x, int32_t src_y,
+		                           int32_t src_w, int32_t src_h,
+		                           int32_t dst_x, int32_t dst_y,
+		                           uint16_t pixel);
+
 	public:
 		DrawContext(int32_t width, int32_t height);
 		~DrawContext();

src/Core/RenderData/BitmapFont.h

@@ -1,15 +1,27 @@
 #pragma once
 #include <cstdint>
-#include "Image.h"
 
 namespace RenderData
 {
 	struct BitmapFont
 	{
-		Image atlas;
+		const uint8_t* mask_bits;
+		int32_t atlas_width;
+		int32_t atlas_height;
 		int32_t char_w;
 		int32_t char_h;
 		int32_t columns;
 		int32_t first_char;
+
+		BitmapFont()
+			: mask_bits(nullptr),
+			  atlas_width(0),
+			  atlas_height(0),
+			  char_w(0),
+			  char_h(0),
+			  columns(0),
+			  first_char(0)
+		{
+		}
 	};
 }