Lock and test the away-direction sign convention for coord_radial()

[wilkox]

Background

Groundwork for the SBOL forward-aesthetic redesign (#64), to be pinned down while working in the polar code for 0.9.0 rather than discovered during 1.0.0.

The SBOL spec (§5.2) requires reverse-strand glyphs to be vertically (away-axis) flipped relative to the forward-strand variant. That flip is only correct if the away-direction sign convention is consistent across coordinate systems:

away+ = "above" in Cartesian = "to the right of" in flipped Cartesian = radially outward in polar

i.e. the forward strand is always on the away+ side, and radially outward in polar coordinates.

Proposal

• Fix and document the away-direction sign convention explicitly.
• Add snapshot/visual-regression coverage that exercises the away+ side in polar coordinates, so an inconsistency here is caught by tests rather than surfacing later as a mysteriously mirrored SBOL glyph.

Why now (0.9.0)

No forward-aesthetic API work is required — just the convention and its tests. Settling it while already in the coord_radial() polar code is far cheaper than discovering a mismatch while building SBOL glyphs in 1.0.0.

Ref #64.