manhattanize the connection lines

[arun3688]

Purpose

GUI: manhattanize steered connections into the target port

Add manhattanize() to snap a connection route to horizontal/vertical
segments like OMEdit: ports stay fixed and the waypoint next to the end
port is moved onto the port's row/column, so the last leg is straight.

Apply it when a connection drawn by dragging is released on a port and
when waypoints of an existing connection are dragged, inserted or removed.
Connections without waypoints keep the default route; files are not
modified on load.

Co-Authored-By: Claude Sonnet 5.5 noreply@anthropic.com

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 27.57%. Comparing base (100fbc8) to head (b2a8500).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1667   +/-   ##
=======================================
  Coverage   27.57%   27.57%           
=======================================
  Files          72       72           
  Lines       13917    13917           
  Branches     8982     8982           
=======================================
  Hits         3838     3838           
  Misses       8865     8865           
  Partials     1214     1214           

Flag	Coverage Δ
linux	27.57% <ø> (ø)

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