`ShaderRef::Handle` missing does not generate warning.

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

If someone used `ShaderRef::Handle` and somehow did not load the handle by mistake, bevy will not generate any error, even in debug mode. The associated meshes simply will not be rendered, which is quite confusing.

## What solution would you like?

This is quite tricky as shaders may take time to load. I think give a frame of grace period, then check if the ID is loading, if not print a warning.

## What alternative(s) have you considered?

Do not generate errors.

Start by tracing the ShaderRef::Handle loading and rendering path in Bevy, focusing on how missing handles differ from shaders that are still loading. Define the one-frame grace period and warning behavior, then verify that valid loading shaders remain quiet and missing handles produce a warning without preventing later rendering.

Written by the indexing model from the issue text.