feat(g.Curve): add isLine() and use it in getSubdivisions()

[kumilingus]

Summary

Closes the @jointjs-dev board card "g: add g.Curve.isLine method".

• g.Curve#isLine() — true when both control points lie on the start–end chord (the curve traces a straight line, possibly overshooting the chord). Point-like curves and loops (start equal to end) return false.
• Tolerant to float noise. The cross product is compared against 1e-9 × chord² rather than exact zero. Since cross = chord length × distance of the control point from the chord, this is a scale-independent "distance below 1e-9 × chord length" test. Motivation: the exact-zero check misses 1913 / 2000 straight curves produced by Curve.throughPoints([a, b]) — the library's own straight-curve constructor — because of the (2·P0 + P3) / 3 arithmetic. With the tolerance: 0 / 5000 missed.
• getSubdivisions() now calls isLine() for its "straight-line curve" special case (forces 2 × precision iterations, because the observed-length convergence test cannot distinguish iterations on a straight curve). Effects:
  • straight curves from throughPoints() get the intended 64 subdivisions (precision 3) instead of 4 → finer t ↔ length mapping for unevenly spaced control points;
  • loops whose start equals end no longer trigger the special case (they were matched by the exact check because cross(start, end) is trivially 0 when start === end).
• No isPoint() alias (the card's suggestion); it would be !isDifferentiable() and no other shape has one.
• No tolerance option — can be added later without breaking; no consumer needs it today.

Changesets: minor for the new method, patch for the getSubdivisions() behaviour change.

Test plan

• karma:geometry — 353/353; new isLine() module (evenly/unevenly spaced, control points equal to endpoints, overshooting, diagonal; curved, one control point off, point, loop; 100 random throughPoints() curves, tiny coordinates, visible deviation rejected). Watched failing first (isLine is not a function). Existing getSubdivisions() assertions (64 for straight, 4 for curved) unchanged.
• karma:joint — 2113/2113
• grunt test:ts
• eslint on all touched files

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