feat: Shadow pass——主光方向光阴影（RenderGraph 双 pass 管线首个验证案例）

- RenderGraph 声明 ShadowMap 虚拟资源（D32，2048²固定分辨率，独立于
  swapchain 尺寸），Shadow pass 写深度 → Forward 声明 ShaderRead 采样，
  barrier/layout（DepthAttachment→ShaderReadOnly）全部由图自动预算
- Shadow pass：空 fragment shader + 深度偏移（斜率缩放）抗 shadow acne
- Forward：UBO 增加 lightSpace 矩阵；Set 2 采样阴影图，手动对比 +
  斜率缩放偏差；clamp-to-border 白色边界（光视锥外 = 全亮）
- 光空间矩阵：主光方向反推 30 单位正交盒；深度重映射 [-1,1]→[0,1]
- GetPhysicalView(slot)：物理镜像按帧槽位隔离，描述符 Compile 后一次性写入
- 修复 RG 隐蔽 bug：ShaderStage 与 PipelineStage2 位值不同
  （eFragment=0x10 vs eFragmentShader=0x80），位值直转会撞出
  TESSELLATION_CONTROL stage；改为逐位显式映射
- 修复 Camera 投影深度未重映射到 [0,1] 的潜在裁剪问题

validation 零错误零警告（3 passes / 5 barriers / 4 gpu images）

shaders/simple_shader.frag

@@ -10,18 +10,42 @@ struct Light {
 layout(location = 0) in vec2 fragUV;
 layout(location = 1) in vec3 fragNormal;
 layout(location = 2) in vec3 fragWorldPos;
+layout(location = 3) in vec4 fragLightPos;
 
 layout(location = 0) out vec4 outColor;
 
 layout(set = 0, binding = 0) uniform UniformBufferObject {
     mat4 view;
     mat4 proj;
+    mat4 lightSpace;
     vec4 ambientColor;
     Light lights[MAX_LIGHTS];
 } ubo;
 
 layout(set = 1, binding = 0) uniform sampler2D texSampler;
 
+// 阴影深度图：手动对比（非 comparison sampler，逻辑清晰且免 format feature 依赖）
+layout(set = 2, binding = 0) uniform sampler2D shadowMap;
+
+// 主光（lights[0]）阴影因子：出光视锥外/未被遮挡 = 1.0
+float ShadowFactor(vec3 N, vec3 L)
+{
+    vec3 proj = fragLightPos.xyz / fragLightPos.w;
+    proj = proj * 0.5 + 0.5;   // NDC [-1,1] -> UV [0,1]
+
+    if (proj.x < 0.0 || proj.x > 1.0 ||
+        proj.y < 0.0 || proj.y > 1.0 ||
+        proj.z > 1.0) {
+        return 1.0;   // 光视锥外：不施加阴影（边界由 border color 兜底）
+    }
+
+    // 斜率缩放偏差，缓解 shadow acne
+    float bias = max(0.004 * (1.0 - dot(N, L)), 0.0008);
+    float closest = texture(shadowMap, proj.xy).r;
+
+    return (proj.z - bias > closest) ? 0.25 : 1.0;
+}
+
 void main() {
     vec3 albedo = texture(texSampler, fragUV).rgb;
     vec3 N = normalize(fragNormal);
@@ -31,8 +55,9 @@ void main() {
     for (int i = 0; i < MAX_LIGHTS; ++i) {
         vec3 L = normalize(-ubo.lights[i].direction.xyz);
         float diff = max(dot(N, L), 0.0);
-        result += ubo.lights[i].color.rgb * ubo.lights[i].color.a * diff * albedo;
+        float shadow = (i == 0) ? ShadowFactor(N, L) : 1.0;
+        result += ubo.lights[i].color.rgb * ubo.lights[i].color.a * diff * albedo * shadow;
     }
 
     outColor = vec4(result, 1.0);
-}
+}