统一 2D 渲染链路以消除格式分叉和热路径中转

将图片主路径收口为 RGBA5551 输入、RGB565 backbuffer 输出，并删除 USE_RGB565_BACKBUFFER 双路径。sprite/tilemap 主绘制改为直接 RGBA5551 -> RGB565，透明语义固定为 1-bit alpha test。同步收口 SDL/FB present 路径、sprite 资源加载/导出规范，以及相关测试与文档。

Constraint: 运行时图片主路径需统一到 RGBA5551，不能继续保留主要 RGBA8888 分支
Constraint: 透明仅支持 1-bit 语义，不引入 alpha blending
Rejected: 保留 USE_RGB565_BACKBUFFER 宏双路径 | 持续增加维护成本并掩盖真实主链路
Rejected: 主绘制继续走 RGBA5551 -> RGBA8888 -> RGB565 | 热路径存在不必要中转
Confidence: high
Scope-risk: moderate
Directive: 后续图片资源默认按 RGBA5551 接入；不要重新引入通用 RGBA8888 主运行时路径
Directive: 字体 atlas 目前仍属兼容输入；若继续收口，优先改成 1-bit mask 而不是新的图片分支
Tested: render_pipeline_tests；SDL Demo Release 构建；搜索确认无 USE_RGB565_BACKBUFFER 残留
Not-tested: Linux framebuffer 目标实机构建与显示验证

src/Apps/Game/tools/asset_pipeline/SpriteAssetTool.cpp

@@ -271,12 +271,25 @@ namespace
 		return atlas_pixels;
 	}
 
-	static void WritePixel(std::ofstream& file, uint32_t pixel)
+	static uint16_t rgba8888_to_rgba5551(uint32_t c)
+	{
+		uint32_t r = (c >> 24) & 0xFFu;
+		uint32_t g = (c >> 16) & 0xFFu;
+		uint32_t b = (c >> 8) & 0xFFu;
+		uint32_t a = c & 0xFFu;
+		return static_cast<uint16_t>(
+			((r >> 3) << 11) |
+			((g >> 3) << 6) |
+			((b >> 3) << 1) |
+			(a ? 1u : 0u));
+	}
+
+	static void WritePixel5551(std::ofstream& file, uint16_t pixel)
 	{
 		file << "0x"
 			<< std::uppercase
 			<< std::hex
-			<< std::setw(8)
+			<< std::setw(4)
 			<< std::setfill('0')
 			<< pixel
 			<< std::dec
@@ -305,7 +318,7 @@ namespace
 		file << "{\n";
 		file << "\tstatic const int32_t tom_atlas_width = " << AtlasWidth << ";\n";
 		file << "\tstatic const int32_t tom_atlas_height = " << atlas_height << ";\n\n";
-		file << "\tstatic const uint32_t tom_atlas_pixels[] = {\n";
+		file << "\tstatic const uint16_t tom_atlas_pixels[] = {\n";
 
 		for (size_t i = 0; i < atlas_pixels.size(); i += 12)
 		{
@@ -313,7 +326,7 @@ namespace
 			const size_t end = std::min(i + 12, atlas_pixels.size());
 			for (size_t j = i; j < end; ++j)
 			{
-				WritePixel(file, atlas_pixels[j]);
+				WritePixel5551(file, rgba8888_to_rgba5551(atlas_pixels[j]));
 				file << ", ";
 			}
 			file << "\n";
@@ -321,7 +334,8 @@ namespace
 
 		file << "\t};\n\n";
 		file << "\tstatic const RenderData::Image image("
-			<< "tom_atlas_pixels, tom_atlas_width, tom_atlas_height, 0x00000000u);\n\n";
+			<< "tom_atlas_pixels, tom_atlas_width, tom_atlas_height, 0x0000, "
+			<< "RenderData::PixelFormat::RGBA5551);\n\n";
 
 		for (size_t i = 0; i < regions.size(); ++i)
 		{