feat(gui): make render thread count configurable at runtime

feat(gui): make render thread count configurable at runtime

Replace the hardcoded NUM_RENDER_SEGMENTS constant with a dynamic
numRenderSegments field on RenderingContext. ViewPanel now exposes
getNumRenderThreads() and setNumRenderThreads() so callers (e.g. the
benchmark) can vary parallelism at runtime.

The render executor is recreated lazily when the thread count changes,
and the rendering context is rebuilt when the segment count no longer
matches. DeveloperToolsPanel displays the active thread count alongside
the available core count.

Also move Sixth 3D Demos TODO items into TODO.org.