fix(connection): symmetric bezier handles and NaN-safe rectangle offset - #291

Fixes two geometry bugs in connection rendering:

1. Asymmetric bezier handles for opposite orientations (fixes #281)

controlPointVertical was computed from controlPoint before the *= 0.5 scale, so when SourceOrientation != TargetOrientation only one of the two Bézier handles was halved — the other kept the full-length vector, producing an asymmetric curve.

Root cause: System.Windows.Vector is a struct; assigning it copies by value. The vertical copy captured the pre-halved value.

// Before fix
Source=(0,0) Target=(200,100) SourceOrientation=Vertical, TargetOrientation=Horizontal
  expected  p1=(0,50)   p2=(150,100)   (both handles = 50)
  actual    p1=(0,100)  p2=(150,100)   (source handle = 100, target handle = 50)

Fix: derive controlPointVertical after the scale so both handles share the same magnitude.

2. NaN offset in ConnectionOffsetMode.Rectangle for axis-aligned connections (fixes #284)

For a horizontal connection with the default offset Size(14, 0), the face-selection condition evaluates to 0 < 0 (false), so the vertical branch runs and computes:

result.Y = Sign(0) * 0   = 0
result.X = 1/Tan(0) * 0  = Infinity * 0  = NaN

WPF draws nothing for a non-finite geometry and raises no exception, making the connection invisibly disappear.

Fix: change < to <= so ties (both sides zero) prefer the horizontal face; add an IsFinite guard to catch any remaining degenerate combos.