`cargo run` fails on WebAssembly target due to `CARGO_BUILD_TARGET` leaking into `rustc_codegen_spirv` build

### Summary
When running the standard `cargo run` development workflow, the build fails while compiling the web frontend (`wasm-pack build ./wrapper --dev --target=web`).

The failure occurs in the `raster-nodes-shaders` crate's `build.rs` script. The build script attempts to compile the `rustc_codegen_spirv` compiler backend, but the parent `CARGO_BUILD_TARGET=wasm32-unknown-unknown` environment variable is inherited by the inner `cargo` build. Because WebAssembly does not support dynamic libraries (`dylib`), the inner build fails.

### Error Output
```text
[INFO]: 🎯 Checking for the Wasm target...
[INFO]: 🌀 Compiling to Wasm...
Compiling raster-nodes-shaders v0.1.0 (../Graphite/node-graph/nodes/raster/shaders)
error: failed to run custom build command for `raster-nodes-shaders v0.1.0 (../Graphite/node-graph/nodes/raster/shaders)`

Caused by:
...
error: cannot produce dylib for `rustc_codegen_spirv v0.9.0 (https://github.com/Firestar99/rust-gpu-new?rev=c12f2161...#c12f2161)` as the target `wasm32-unknown-unknown` does not support these crate types
```

### Steps to Reproduce
1. Clone the repository.
2. Ensure Node.js and Rust prerequisites are installed.
3. Run `cargo run`.
4. Observe the build crash when `wasm-pack` reaches `raster-nodes-shaders`.

### Proposed Solution
The inner build initiated by `cargo_gpu::Install` inside `node-graph/nodes/raster/shaders/build.rs` needs to compile for the host OS, not the WebAssembly target.

Clearing the `CARGO_BUILD_TARGET` environment variable inside `build.rs` before it kicks off the `cargo_gpu` build resolves the issue and allows the compilation to succeed:

```rust
// node-graph/nodes/raster/shaders/build.rs

// Inside main():
unsafe { std::env::remove_var("CARGO_BUILD_TARGET"); }

// ... followed by the existing cargo_gpu::Install::from_shader_crate(...) logic
```

### Environment
* **OS:** macOS 26.3.1 (Build 25D2128)
* **Rust Version:** rustc 1.92.0 (ded5c06cf 2025-12-08)
* **Node.js:** v22.21.1

### 🤖 AI disclosure

I used Gemini to help me understand the error message. This message and the fix are mine.