fix(visualization): keep layer arrow in frame for linear flows

[shantoshdurai]

Fixes #535

Linear open graphs place every node on one row, so ymin == ymax. Layer labels use fixed point offsets below plot_lims.ymin, but the ymin adjustment in _draw_layers ran before axis limits were set, so the data/display transform was degenerate and the "Layer" arrow ended up below the plot frame.

Changes

• Set preliminary plot limits before _draw_layers so point-based offsets map correctly
• Expand ymin to include layer label height (va='top') plus padding
• Enforce a minimum vertical axis span in _set_plot_lims when node rows collapse to one line
• Add regression test test_linear_causal_flow_layer_label_in_frame

Verify

pytest tests/test_visualization.py::test_linear_causal_flow_layer_label_in_frame -q

[codecov]

Codecov Report

❌ Patch coverage is 78.57143% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.83%. Comparing base (af1c16d) to head (03302b3).

Files with missing lines	Patch %	Lines
graphix/visualization.py	78.57%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #536      +/-   ##
==========================================
- Coverage   88.85%   88.83%   -0.03%     
==========================================
  Files          49       49              
  Lines        7135     7146      +11     
==========================================
+ Hits         6340     6348       +8     
- Misses        795      798       +3     

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