fix: PNG backend twin axes top-axis tick label overlaps plot title (fixes #1700)

[krystophny]

Summary

When twiny() creates a secondary x-axis at the top of the plot, the figure title was rendered at a fixed vertical offset from the plot area boundary, causing it to overlap with the top-axis tick labels and the top xlabel ("Cumulative index (log scale)").

The fix modifies compute_title_position_sized in fortplot_raster_labels.f90 to account for last_x_tick_max_height_top (set by raster_draw_x_axis_ticks_top) when positioning the title. When twiny is active (indicated by last_x_tick_max_height_top > 0), the title is placed above the top xlabel rather than at the fixed offset.

Changes

• src/backends/raster/fortplot_raster_labels.f90: Added top_tick_height parameter to compute_title_position_sized and compute_title_position. When top_tick_height > 0, the title y position is computed to sit above the top-axis xlabel.
• test/test_title_position_twiny.f90: New regression test verifying title positioning with and without twiny.

Verification

Test passes after fix

$ fpm test --target test_title_position_twiny
 PASS: Title positioned correctly with and without twiny

$ fpm test --target test_title_centering_raster
 PASS: Title centering matches expected pixel position

$ fpm test 2>&1 | grep -c "PASS:"
827  (all tests pass, no failures)

$ make verify-artifacts
Artifact verification passed.

Artifact evidence

• output/example/fortran/twin_axes_demo/twin_axes_demo.png (49299 bytes) regenerated with fix
• output/example/fortran/twin_axes_demo/twin_axes_demo.pdf (6246 bytes) regenerated with fix
• Text output confirms title and top xlabel render at distinct positions:

  Twin axis demo
  +--------------------------------------------------------------------------------+
  |      1                                Cumulative index (log scale)             |