fix(m2): degenerate-normal guard during walkable-floor raycast

The matrix-transformed normal could be near-zero if the M2 instance
has a degenerate scale; glm::normalize then returns NaN that
contaminates the slope check (NaN < 0.35 is false → no early-out)
and bestNormalZ goes NaN, breaking the walkable-floor heuristic.

Length-check the transformed normal and fall back to the (0,0,1)
flat default — same pattern as the WMO renderer.

src/rendering/m2_renderer_instance.cpp

@@ -765,8 +765,12 @@ std::optional<float> M2Renderer::getFloorHeight(float glX, float glY, float glZ,
                 if (nLen > 0.001f) {
                     localN /= nLen;
                     if (localN.z < 0.0f) localN = -localN;
-                    worldN = glm::normalize(
-                        glm::vec3(instance.modelMatrix * glm::vec4(localN, 0.0f)));
+                    glm::vec3 transformedN = glm::vec3(
+                        instance.modelMatrix * glm::vec4(localN, 0.0f));
+                    float wnLen = glm::length(transformedN);
+                    if (wnLen > 0.001f) {
+                        worldN = transformedN / wnLen;
+                    } // else: keep worldN = (0,0,1) flat default
                     if (std::abs(worldN.z) < 0.35f) continue; // too steep (~70° max slope)
                 }