Skybox falls back to a mode that binds a null cubemap #14

When a panorama load fails, Skybox logs a warning, switches m_mode to Gradient and still sets m_initialized (Skybox.cpp:110-116). Gradient mode never creates m_cubemapTexture, but Render only checks shader validity (:125) and the command lambda captures the null RefPtr into ctx->BindTexture at :163.

DebugLineRenderer has the same shape: Initialize returns early on shader failure before s_vertices is allocated (:32 vs :50), and DrawLine's first statement dereferences s_vertices with no null check (:67). Flush and Shutdown both check; DrawLine forgot.