Stagger pin labels in staircase layout

[Ntrondle]

Summary
place top/bottom pin labels on separate stepped levels
extend their leader lines to match each label level
keep left/right pin labels vertically aligned because they are already separated
expand the SVG padding so staggered labels are not clipped
keep the Python generator and browser designer renderers in sync

Why
Dense connectors previously placed labels on one shared baseline and reduced the font size to fit the available pin pitch. This made adjacent labels difficult to distinguish and increased the risk of associating a definition with the wrong pin.

The staircase layout preserves a readable label size and gives horizontally arranged pins visually distinct leaders. Connectors whose labels already form a vertical list keep equal leader lengths.

Validation
python -m compileall -q pinout_gen/pinout_gen
node --check pinout_design/js/svg-renderer.js
generated pinout_gen/example.toml successfully
checked that top/bottom orientations stair-step and left/right orientations remain aligned
checked a two-row connector
visually inspected the generated HTML

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[xbst]

Thanks for implementing this, helps a lot with long pin names on horizontal connectors. Merged.