layout: add shape: cycle with circular border-trimmed arc routing

[Visio-Vanitas]

Description

This PR implements shape: cycle for circular diagram layouts with circular Bézier arc routing that accurately terminates at visible shape boundaries (fixing #1578, resolving the open constraints from #2362).

Implementation Details

1. Circular Bézier Arc Representation:

   • Arcs are partitioned into sub-segments ($\Delta\theta \le \pi/2$) and represented as cubic Bézier curves with the optimal control point distance factor $k = \frac{4}{3} \tan\left(\frac{\Delta\theta}{4}\right)$.
   • Conforms strictly to D2's curve route contract ($1 + 3n$ points).
   • Because the tangent vector at the endpoint is collinear with the circular arc tangent, arrowhead alignment at the boundary is naturally oriented without artificial distortion.

2. Accurate Shape Boundary Trimming:

   • Evaluates the circular trajectory against shape.Shape.Perimeter().Intersections(), accurately snapping the arc entry/exit points to the visible perimeter of diverse shapes (rectangle, circle, cylinder, hexagon, etc.).
   • Includes binary search boundary resolution as fallback for complex geometry bounds.

3. Cycle Configurations & Nested Container Support:

   • Single-node cycles ($N=1$) are centered gracefully with radius 0 (preventing division-by-zero on $\sin(\pi)$).
   • Nested child containers and descendant edges (a.x -> b.y) preserve their relative coordinates and connections during cycle container layout.

4. Testing & Golden Fixtures:

   • Added unit test suite in d2layouts/d2cycle/layout_test.go covering radius computation, single-node safety, nested layout preservation, cross-child descendant routes, and shape diversity.
   • Added E2E txtar test cases in e2etests/txtar.txt under -- cycle-diagram --. Generated golden .exp.json and .exp.svg files across Dagre, ELK, and Tala layout engines via TA=1 ./ci/test.sh ./e2etests -run TestE2E/txtar/cycle-diagram -v (all pass).

Closes #1578
/claim #1578