Refactor Jacobian assembly

[KakeruUeda]

Description

Jacobian assembly currently uses a COO matrix, and we sort and rebuild CSR row pointers each step for IDA. The overhead from this process can become a bottleneck as nnz grows. This change keeps the system Jacobian in CSR and preserves a COO→CSR mapping for direct CSR updates.

Closes #301

cc @nkoukpaizan

Proposed changes

• Store the system Jacobian in CSR format in PowerElectronicsModel.
• Update allocate() to populate CSR entries and preserve the mapping.
• Add CsrJac for the solver configuration (PhasorDynamics still uses the Jac function).

Currently PowerElectronics only.

Checklist

• All tests pass.
• Code compiles cleanly with flags -Wall -Wpedantic -Wconversion -Wextra.
• The new code follows GridKit™ style guidelines.
• [N/A] There are unit tests for the new code.
• The new code is documented.
• [N/A] The feature branch is rebased with respect to the target branch.
• I have updated CHANGELOG.md to reflect the changes in this PR. If this is a minor PR that is part of a larger fix already included in the file, state so.