PositionPlane init args don't match the axis pair that enables each plane

[manaporkun]

Spun off from review on #13 / #14.

In PositionHandle.Initialize, each PositionPlane is initialized so that the plane it constrains to does not match the axis pair that enables it. Concretely, on `main` today:

Plane GameObject	Enabled when	Initialize args (axis1, axis2, perp)	Actual constraint plane	Expected for the enabling pair
zPlane	HasBothAxes(X, Y)	(forward, up, -right)	YZ (normal = X)	XY (normal = Z)
xPlane	HasBothAxes(Y, Z)	(right, forward, up)	XZ (normal = Y)	YZ (normal = X)
yPlane	HasBothAxes(X, Z)	(right, up, forward)	XY (normal = Z)	XZ (normal = Y)

So when the user selects (say) HandleAxes.XY and drags the plane gizmo, the target moves in the YZ plane instead of XY.

This is an existing bug on main; #14 leaves it alone to keep that PR focused on #10.

Two ways to fix:

1. Adjust Initialize args so each plane constrains to the plane indicated by the enabling axis pair. This also moves the visual quad (_quadGameObject.localPosition = (axis1+axis2)*0.2) to sit between the two enabling axes — which is what users would expect visually.
2. Adjust the enabling conditions so each plane GameObject activates when the axis pair matching its current constraint is selected. Keeps the existing visual positions but is less intuitive in naming.

Recommend option 1.

[manaporkun]

Resolved on develop (PR #22), commit c5a0428.

PositionHandle.Initialize now enables each plane on the axis pair its geometry/perp actually represents, so the drag constraint matches the active plane:

Plane	Active when	perp	Drag plane
zPlane	YZ	-X	YZ
xPlane	XZ	Y	XZ
yPlane	XY	Z	XY

The drag plane is new Plane(rPerp, pos) (PositionPlane.StartInteraction), so its normal = perp and motion is confined to the two in-plane axes. Each active plane now moves the target only in its own plane. Closes once PR #22 merges to main.