Add a MacOS builder

Nobody has claimed this yet.

After #267 Lighthouse is supposed to run on MacOS, but we need a CI builder to make sure it works consistently and doesn't regress.

I've added `macos-26` to the matrix in https://github.com/llvm/lighthouse/tree/macos, but the error in Torch means we'll have to also update the `toml` file somehow.

```
error: Distribution `torch==2.13.0+cpu @ registry+[https://download.pytorch.org/whl`](https://download.pytorch.org/whl%60) can't be installed because it doesn't have a source distribution or wheel for the current platform
hint: You're on macOS (`macosx_26_0_arm64`), but `torch` (v2.13.0+cpu) only has wheels for the following platforms: `manylinux_2_28_aarch64`, `manylinux_2_28_x86_64`, `linux_s390x`, `win_amd64`, `win_arm64`; consider adding "sys_platform == 'darwin' and platform_machine == 'arm64'" to `tool.uv.required-environments` to ensure uv resolves to a version with compatible wheels
Error: Process completed with exit code 2.
```

Torch-MLIR also doesn't have support for MacOS (only `manylinux` and `win`), so that's another thing that we'll need to fix before we can officially claim to support MacOS.

Inspect the CI matrix on the macos branch and the project's TOML dependency configuration; start by reproducing the macos-26 arm64 workflow and its uv resolution error. Done means the MacOS builder completes consistently with compatible Torch and Torch-MLIR dependencies and remains in CI.

Written by the indexing model from the issue text.