fix: Mark get_log_level() pure for consistency (fixes #1347) (#1348)

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).

Author: krystophny

src/external/fortplot_logging.f90

@@ -12,7 +12,8 @@ module fortplot_logging
     private
 
     public :: set_log_level, get_log_level, log_info, log_warning, log_error, log_debug
-    public :: LOG_LEVEL_SILENT, LOG_LEVEL_ERROR, LOG_LEVEL_WARNING, LOG_LEVEL_INFO, LOG_LEVEL_DEBUG
+    public :: LOG_LEVEL_SILENT, LOG_LEVEL_ERROR, &
+              LOG_LEVEL_WARNING, LOG_LEVEL_INFO, LOG_LEVEL_DEBUG
     public :: initialize_warning_suppression, is_warnings_suppressed
 
     ! Log levels (in increasing verbosity)
@@ -47,7 +48,7 @@ subroutine set_log_level(level)
         current_log_level = lvl
     end subroutine set_log_level
 
-    function get_log_level() result(level)
+    pure function get_log_level() result(level)
         !! Get the current global logging level
         !! Returns one of: LOG_LEVEL_SILENT, LOG_LEVEL_ERROR,
         !!                  LOG_LEVEL_WARNING, LOG_LEVEL_INFO, LOG_LEVEL_DEBUG
@@ -120,7 +121,8 @@ subroutine initialize_warning_suppression()
         end if
 
         ! Check for manual warning suppression override
-        call get_environment_variable('FORTPLOT_SUPPRESS_WARNINGS', env_value, status=status)
+        call get_environment_variable('FORTPLOT_SUPPRESS_WARNINGS', env_value, &
+                                      status=status)
         if (status == 0 .and. len_trim(env_value) > 0) then
             warnings_suppressed = parse_boolean_env(env_value)
             suppression_initialized = .true.