Angle2D/Arc2D/SmallestArc2D render wrong under canvas scale

[bewuwy]

Problem

Angle2D (and Arc2D / SmallestArc2D, which wrap it) only sends its origin through Projection2D. The start and end angles (points[i].angle() in Arc2D.svelte) are world angles and are used as given, while the vectors themselves are drawn along their projected on-screen directions. Under a non-uniform Canvas2D scale (scaleX !== scaleY) the arc ends therefore do not land on the drawn vectors.

SmallestArc2D's label position is the world-space bisector times distance, and Latex2D projects it. The arc radius (distance) is a screen-space size that is not projected, so the label drifts off the arc whenever the scale is not 1, even when it is uniform.

Expected

• The arc starts and ends on the on-screen direction of each vector.
• The label sits on the drawn arc.
• Output at scale 1 is unchanged.

Suggested approach

• Add a world angle → screen angle mapping to Projection2D (atan2(sy·sinθ, sx·cosθ), lifted so winding is kept for negative and multi-turn angles), with unit tests.
• Use it in Angle2D for the start and end angles and the arrowhead rotation. Keep distance, width and headLength as screen-space sizes, as documented in Angle2D now.
• Compute SmallestArc2D's label position in screen space and pass it back through projection.toWorld.

Context

Found in the review of #515 (the Projection2D seam, #491) and deferred from that PR.