fix: PNG backend: markers at data range boundary clipped by plot border (fixes #1683)

[krystophny]

Summary

Increase the data-range margin expansion from 2% to 5% so markers placed at the exact data-range boundary are pushed far enough inside the plot area to avoid visual overlap with the axes frame.

Changes

• src/figures/fortplot_figure_rendering_pipeline.f90: bump PDF_DATA_RANGE_MARGIN from 0.02_wp to 0.05_wp
• test/test_marker_boundary_clipping.f90: new regression test placing markers at all four data-range corners and verifying each has sufficient visible pixel coverage

Root Cause

The expand_data_range subroutine inflates the coordinate system so boundary data points map inside the plot area rather than onto the axes frame. A 2 % inflation on a typical ~400 px plot height gives only ~8 px of clearance, which is barely enough for a 5–7 px marker radius. At that distance the 1 px axes frame line overlaps the outer edge of the marker, making it look "half-clipped".

A 5 % inflation gives ~20 px clearance, which comfortably accommodates all built-in marker sizes.

Verification

Test passes after fix

$ fpm test --target test_marker_boundary_clipping
 PASS: all four corner markers fully visible
   corner pixel counts:         699         521         463         408

Full test suite green

$ fpm test 2>&1 | grep -c "STOP 1\|<ERROR>"
0

Affected examples regenerate cleanly

• make example ARGS="scatter_demo" → scatter_basic.png, scatter_multi.png, scatter_gaussian.png
• make example ARGS="disconnected_lines" → disconnected_lines.png, disconnected_lines.pdf

Artifact file sizes are consistent with pre-change renders (no regression in output volume).