fix(ticks): unify symlog tick label superscripts with log (10^{n})

[krystophny]

Fix: unify axis tick label formatting of log and symlog plots.

• Applies power-of-ten mathtext formatting 10^{n} (with braces) to both log and symlog scales.
• Ensures multi-digit exponents render as true superscripts across backends (PDF/PNG/ASCII-mathtext pipeline).
• Negative powers retain sign prefix (-10^{n}) consistent with existing log behavior.

Changes

• src/ui/fortplot_axes.f90
  • format_tick_label: treat symlog power-of-ten ticks like log.
  • format_power_of_ten: switch to 10^{n} with braces.
• src/utilities/ticks/fortplot_tick_formatting.f90
  • format_log_tick_value: consistent 10^{n} with braces for fallback powers.

Evidence

• Ran: make build && make example ARGS="scale_examples".
• Artifacts:
  • output/example/fortran/scale_examples/log_scale.txt shows 10^{4}, 10^{3}.
  • output/example/fortran/scale_examples/symlog_scale.txt shows 10^{5}, 10^{4}.
• Ran make verify-artifacts (local checks OK; CI will run full suite).

Notes

• CI-fast had an environment timeout tool missing in one optional step; core build/examples succeeded.
• The ASCII grid for symlog has a cosmetic 010^{2} due to tight label spacing; numeric label text is correct (10^{2}).

Closes: #