Description:
When specifying platform-specific dependencies in requirements.in for optimum[onnxruntime] and optimum[onnxruntime-gpu], the generated requirements.txt file consolidates dependencies inappropriately. Specifically, optimum[onnxruntime-gpu]; sys_platform == 'linux' and optimum[onnxruntime]; sys_platform == 'darwin' results in optimum[onnxruntime,onnxruntime]==1.18; sys_platform == 'linux' in the output requirements.txt, which causes the Darwin platform dependency to be ignored.

Steps to Reproduce:

1. Create a requirements.in file with the following lines:

   optimum[onnxruntime-gpu]; sys_platform == 'linux'
   optimum[onnxruntime]; sys_platform == 'darwin'
   

2. Run uv pip compile --universal requirements.in to generate requirements.txt.
3. Open the resulting requirements.txt.

Expected Behavior:
requirements.txt should list both platform-specific dependencies separately, like:

optimum[onnxruntime-gpu]==1.18; sys_platform == 'linux'
optimum[onnxruntime]==1.18; sys_platform == 'darwin'

Actual Behavior:
The generated requirements.txt incorrectly merges dependencies, resulting in:

optimum[onnxruntime,onnxruntime-gpu]==1.18; sys_platform == 'linux'

This line omits the macOS (Darwin) dependency entirely.

Environment:

• uv version: 0.4.30

• Operating System: Linux