docs: gpu_solver_jacobian.md historical note references non-existent analysis/dc.py

[github-actions]

File: docs/gpu_solver_jacobian.md (line 7)

Problem: The historical note says:

DC analysis is now in analysis/dc.py, transient in analysis/transient/, and all device models use OpenVAF analytical Jacobians.

But analysis/dc.py does not exist. The actual DC analysis module is analysis/dc_operating_point.py.

```
$ ls vajax/analysis/dc*
vajax/analysis/dc_operating_point.py # exists

vajax/analysis/dc.py # does NOT exist

```

This is the same class of error as #98 (which covers gpu_solver_architecture.md having the same wrong path).

Fix: Replace analysis/dc.py with analysis/dc_operating_point.py in the historical note.