Shadow visibility term is non-perceptual.

## Bevy 0.18

Bevy just uses the shadow visibility term directly. The resulting gradient is not perceptually linear so it ends up looking blocky. Should use something like [hardenedKernel](https://github.com/google/filament/blob/1f6a67b2a2798acdfbf14ea9aba3686dd5530b2a/shaders/src/surface_shadowing.fs#L118) from Filament.

Current bevy 0.18/main. Raw visibility term.

[hardenedKernel](https://github.com/google/filament/blob/1f6a67b2a2798acdfbf14ea9aba3686dd5530b2a/shaders/src/surface_shadowing.fs#L118)

pow(visibility, 2.2)

Start at Bevy's shadow visibility term in the rendering code; the issue names no file or test. Compare the current raw visibility output with Filament's hardenedKernel and the pow(visibility, 2.2) example, then validate that the resulting gradient is perceptually smoother without regressing shadow behavior.

Written by the indexing model from the issue text.