fix: Mark get_log_level() pure for consistency (fixes #1347)

[krystophny]

Summary

• Mark get_log_level() as pure since it reads a module variable without side effects.
• Clarifies intent, enables compiler optimizations, and aligns with project guidance to prefer purity when applicable.

Verification
Commands run locally:

• make test-ci
  • CI essential suite completed successfully
• make test
  • ALL TESTS PASSED (fpm test)
• make verify-artifacts
  • Artifact verification passed (no plot/text regressions)

Key excerpts:

• CI essential: "CI essential test suite completed successfully"
• Full suite: "ALL TESTS PASSED (fpm test)"
• Artifacts: strict checks on PDFs/PNGs/txt passed

No artifact/output differences, as expected (signature-only attribute change).

Housekeeping

• Wrapped two long lines in src/external/fortplot_logging.f90 to comply with the 88–90 column limit (no functional changes).