Add shadow frustum culling to terrain and M2 depth passes

Both passes were rendering the entire loaded scene (17×17 tile radius)
into a shadow map that only covers 360×360 world units — submitting
10-50× more geometry than the shadow frustum can actually use.

- TerrainRenderer::renderShadow: skip chunks whose bounding sphere
  doesn't overlap the shadow frustum AABB in XY. Reduces terrain draw
  calls from O(all loaded chunks) to O(chunks within ~180 units).
- M2Renderer::renderShadow: skip instances whose world AABB doesn't
  overlap the shadow frustum in XY. Reduces M2 draw calls similarly.
- Both functions now take shadowCenter + halfExtent parameters.

include/rendering/m2_renderer.hpp

@@ -236,7 +236,7 @@ public:
     /**
      * Render depth-only pass for shadow casting
      */
-    void renderShadow(GLuint shadowShaderProgram);
+    void renderShadow(GLuint shadowShaderProgram, const glm::vec3& shadowCenter, float halfExtent);
 
     /**
      * Render smoke particles (call after render())

include/rendering/terrain_renderer.hpp

@@ -135,7 +135,7 @@ public:
     /**
      * Render terrain geometry into shadow depth map
      */
-    void renderShadow(GLuint shaderProgram);
+    void renderShadow(GLuint shaderProgram, const glm::vec3& shadowCenter, float halfExtent);
 
     /**
      * Set shadow map for receiving shadows

src/rendering/m2_renderer.cpp

@@ -2151,7 +2151,7 @@ void M2Renderer::render(const Camera& camera, const glm::mat4& view, const glm::
     }
 }
 
-void M2Renderer::renderShadow(GLuint shadowShaderProgram) {
+void M2Renderer::renderShadow(GLuint shadowShaderProgram, const glm::vec3& shadowCenter, float halfExtent) {
     if (instances.empty() || shadowShaderProgram == 0) {
         return;
     }
@@ -2172,6 +2172,12 @@ void M2Renderer::renderShadow(GLuint shadowShaderProgram) {
     glActiveTexture(GL_TEXTURE0);
 
     for (const auto& instance : instances) {
+        // Cull instances whose AABB doesn't overlap the shadow frustum (XY plane)
+        glm::vec3 instCenter = (instance.worldBoundsMin + instance.worldBoundsMax) * 0.5f;
+        glm::vec3 instHalf   = (instance.worldBoundsMax - instance.worldBoundsMin) * 0.5f;
+        if (std::abs(instCenter.x - shadowCenter.x) > halfExtent + instHalf.x) continue;
+        if (std::abs(instCenter.y - shadowCenter.y) > halfExtent + instHalf.y) continue;
+
         auto it = models.find(instance.modelId);
         if (it == models.end()) continue;
 

src/rendering/renderer.cpp

@@ -3444,9 +3444,10 @@ void Renderer::renderShadowPass() {
     if (useBonesLoc >= 0) glUniform1i(useBonesLoc, 0);
     if (texLoc >= 0) glUniform1i(texLoc, 0);
 
-    // Render terrain into shadow map
+    // Render terrain into shadow map (only chunks within shadow frustum)
     if (terrainRenderer) {
-        terrainRenderer->renderShadow(shadowShaderProgram);
+        glm::vec3 shadowCtr = shadowCenterInitialized ? shadowCenter : characterPosition;
+        terrainRenderer->renderShadow(shadowShaderProgram, shadowCtr, kShadowHalfExtent);
     }
 
     // Render WMO into shadow map
@@ -3476,9 +3477,10 @@ void Renderer::renderShadowPass() {
         shadowShaderWrapper.releaseProgram();  // Don't let wrapper delete our program
     }
 
-    // Render M2 doodads into shadow map
+    // Render M2 doodads into shadow map (only instances within shadow frustum)
     if (m2Renderer) {
-        m2Renderer->renderShadow(shadowShaderProgram);
+        glm::vec3 shadowCtr = shadowCenterInitialized ? shadowCenter : characterPosition;
+        m2Renderer->renderShadow(shadowShaderProgram, shadowCtr, kShadowHalfExtent);
     }
 
     // Render characters into shadow map

src/rendering/terrain_renderer.cpp

@@ -369,7 +369,7 @@ GLuint TerrainRenderer::createAlphaTexture(const std::vector<uint8_t>& alphaData
     return textureID;
 }
 
-void TerrainRenderer::renderShadow(GLuint shaderProgram) {
+void TerrainRenderer::renderShadow(GLuint shaderProgram, const glm::vec3& shadowCenter, float halfExtent) {
     if (chunks.empty()) return;
 
     GLint modelLoc = glGetUniformLocation(shaderProgram, "uModel");
@@ -378,6 +378,13 @@ void TerrainRenderer::renderShadow(GLuint shaderProgram) {
 
     for (const auto& chunk : chunks) {
         if (!chunk.isValid()) continue;
+
+        // Cull chunks whose bounding sphere doesn't overlap the shadow frustum (XY plane)
+        float maxDist = halfExtent + chunk.boundingSphereRadius;
+        float dx = chunk.boundingSphereCenter.x - shadowCenter.x;
+        float dy = chunk.boundingSphereCenter.y - shadowCenter.y;
+        if (dx * dx + dy * dy > maxDist * maxDist) continue;
+
         glBindVertexArray(chunk.vao);
         glDrawElements(GL_TRIANGLES, chunk.indexCount, GL_UNSIGNED_INT, 0);
         glBindVertexArray(0);