Add planar reflections example

## What problem does this solve or what need does it fill?

Sharp and accurate reflections for mirrors or water surfaces.
Easy to Implement.
Much cheaper than ray-traced reflections, making them ideal for low-end hardware or performance-critical applications (e.g., VR, mobile), avoiding the heavy BVH traversal of RT.
Unlike screen-space reflections (SSR), planar reflections work even when reflected objects are off-screen, making them more reliable.
No temporal flickering or disocclusion artifacts common in SSR.
No need for hardware-accelerated RT cores or complex denoising.

## What alternative(s) have you considered?

Wait for RT.

## Additional context

Possible optimizations:
Add dynamic resolution scaling, based on quality settings.
Scene only rendered second time if reflection mask is visible.

No files, tests, or entry points are named. Begin by locating the existing rendering examples and reflection-related entry points, then clarify the intended API, supported surfaces, and quality behavior; done should be a working planar-reflections example with agreed performance and visibility optimizations.

Written by the indexing model from the issue text.