Forward Decals render incredibly weird

## Bevy version and features

Base `0.16`

## \[Optional\] Relevant system information

```
2025-09-05T15:58:10.951233Z INFO bevy_diagnostic::system_information_diagnostics_plugin::internal: SystemInfo { os: "Linux (NixOS 25.05)", kernel: "6.12.34", cpu: "Intel(R) Core(TM) i7-14700KF", core_count: "20", memory: "31.2 GiB" }
2025-09-05T15:58:11.090925Z INFO bevy_render::renderer: AdapterInfo { name: "NVIDIA GeForce RTX 4060 Ti", vendor: 4318, device: 10243, device_type: DiscreteGpu, driver: "NVIDIA", driver_info: "570.153.02", backend: Vulkan }
```

## What you did

https://github.com/hukasu/bevy_min_repro/tree/20888-at-0.16

## What went wrong

Forward Decals render in a perfect square even though there are parts of it that should be rendered on a mesh

## Additional information

Start with the linked bevy_min_repro branch 20888-at-0.16 and reproduce the forward-decal scene on the reported Bevy 0.16 setup. Trace the forward-decal rendering path to determine why the decal appears as a square rather than being limited to the mesh, then verify that the decal is clipped to the intended mesh.

Written by the indexing model from the issue text.