refactor(deltaspin): split lambda loop diagnostics and add ScState tests

[hujieting]

Summary

This PR refactors the DeltaSpin module by separating the lambda scanning diagnostics from the main optimization loop and adding unit tests for ScState::cal_escon().

Changes

• Move run_lambda_linear_scan() from lambda_loop.cpp to the new lambda_loop_diag.cpp.
• Update CMakeLists.txt to include the new source file.
• Reduce lambda_loop.cpp from 621 to 405 lines.
• Add three GoogleTest cases for ScState::cal_escon():
  • CalEsconEmpty
  • CalEsconSingleAtom
  • CalEsconMultiAtom
• Register the new test target in CMake.

Verification

• DeltaSpin module compilation and linking succeeded.
• Existing DeltaSpin tests: 6/6 passed.
• New GoogleTest cases: 3/3 passed.
• Git diff check passed.
• Governance check completed without blocking violations.

Notes

• No numerical algorithms or physical calculations were modified.
• No changes from Task 1 or Task 2 are included.
• The refactoring and unit tests are organized into two separate commits.