d2cycle: route "shape: cycle" edges as circular arcs clipped at shape borders

[kimotostudio]

Fixes #1578

/claim #1578

Builds on the approach in #2362: shape: cycle arranges objects in a circle and routes edges as perfectly circular arcs — and makes the arcs start/stop exactly on the shape borders while staying smooth. No renderer changes; measured worst endpoint-to-border distance 0.001px, anchor-radius spread 0.000px.

How

Instead of sampling the arc as a polyline and clipping it afterwards (which fights the renderer's cubic-Bézier interpretation of IsCurve routes), the route is generated analytically:

• The exact angle where the layout circle crosses each shape's border is found with a chord scan + bisection (~0.001px accuracy). Rectangular shapes clip at their bounding box (they define no Perimeter(), matching shape.TraceToShapeBorder semantics); non-rectangular shapes (circle, hexagon, ...) clip at their visible Perimeter().
• The clipped arc is emitted as standard cubic Bézier segments (k = 4/3·tan(Δθ/4), one segment per quarter turn), i.e. a [P0, C1, C2, P1, ...] route — so d2svg.pathData renders it as-is and no renderer changes are needed.
• Node positions are snapped to integer coordinates so the export-time int() truncation cannot shift boxes off the (float) route endpoints.

Also guards the degenerate cases: single-node cycles no longer produce an infinite radius, and clipping falls back to the center-to-center arc if no border crossing is found.

Testing

• TA=1 ./ci/test.sh ./e2etests -run TestE2E/txtar/cycle-diagram -v (the command from the bounty comment) generates the fixture; dagre + elk both pass. Measured from the generated boards: worst endpoint-to-border distance 0.001px, anchor-radius spread 0.000px (perfectly circular).
• New package tests in d2layouts/d2cycle: border contact, circularity, non-rectangular perimeter clipping, single-node guard.
• CI=1 go test ./... passes with the repo's pinned toolchain (go1.25.0).
• Rendered output matches the target look in the issue (see the cycle-diagram e2e fixture SVGs in this diff).

Known limitations (happy to follow up): nested containers inside a cycle node keep their default layout (same as #2362), and outside labels/icons are not yet obstacles for the arc.

Disclosure: implemented with AI assistance (Claude), reviewed and submitted by me.