fix(example): clean up v10 demos

[VictorZakharov]

Summary

• Replace deprecated state.gl access with state.renderer in the default and WebGPU demos.
• Remove the unreachable Fog demo, which also depended on package-internal source imports.
• Keep state.gl only in the explicitly legacy demos.

Verification

• pnpm run ci (620 tests passed, 9 todo)
• pnpm --filter example build
• Verified every remaining demo module is referenced and no non-legacy demo accesses state.gl.

Closes #3840