Add arc, bezier and helix free functions - #2131

Adds free functions for curves that were previously only available as Edge/Wire classmethods, as discussed in #2126.

arc(p1, p2, p3)                    # through three points
arc(p1, p2, tangent=t)             # tangent at p1
circle(r, start=0, end=360)        # full circle or arc (degrees)
bezier(*pts)
helix(pitch, height, radius, lefthand=False)

Why this differs from the discussion

The discussion proposed arc(r, a1, a2) as a third arc overload. multimethod only dispatches on positional types, so keyword calls (arc(r=1, ...)) and missing arguments failed with an opaque DispatchError. Instead, arc is a plain function for the point-based arcs, and arcs from radius and angles go to circle(r, start, end), mirroring Edge.makeCircle(radius, angle1=, angle2=).

Decisions to review

• arc(p1, p2, p3) and arc(p1, p2, tangent=t) as one plain function; exactly one of p3/tangent is required
• circle(r, start=0, end=360): degrees, counterclockwise, start < end <= start + 360
• helix(pitch, height, radius, lefthand=False): radius (like circle), not diameter (like cylinder); full-word argument names; returns an Edge, not a Wire
• No conical helix yet
• Inputs are validated with explicit ValueError/TypeError (collinear or coincident points, zero or parallel tangent, non-positive sizes, invalid angle ranges). circle(0) now raises instead of returning a degenerate edge
• ellipse is unchanged: its angles would be parametric rather than polar, so partial ellipses are left out for now