Fix the sampled domain of a periodic spline (bounding box reached the origin)

[redbluevn]

The problem

getStartAndEndKnots tests IsClosed before IsPeriodic:

if (this.IsClosed)          { uStart = knots[0];            uEnd = knots[^1]; }
else if (this.IsPeriodic)   { uStart = knots[this.Degree];  uEnd = knots[^(this.Degree + 1)]; }

A periodic spline forms a closed loop and therefore carries the closed flag too, so the periodic branch never runs and the domain is taken as knots[0] .. knots[last].

For an unclamped knot vector the domain starts at knots[degree]. Evaluating below it makes every basis function vanish, and PolygonalVertexes returns the zero vector for that sample.

What it costs

Measured on a periodic spline out of a real drawing — degree 3, 7 control points, 11 knots spanning 0..2, IsClosed = IsPeriodic = true:

CTRLBOX     (4021467.1, -9264141.7) .. (4021541.2, -9264045.6)
SAMPLED     256 points, zeros=1, at index [0]
BOX         (0, -9264141.7) .. (4021541.2, 0)

A spline about 59 by 74 units across reports a box four million by nine million, because one sample came back as (0,0,0) and BoundingBox.FromPoints honoured it. It lives inside a block, so every reference to that block inherited the box, and the drawing's extents with it.

After the fix, zeros=0 and the box is the spline's own (4021482.2, -9264138.8) .. (4021541.2, -9264067.2).

The fix

Ask about periodic first. The knot vector decides the domain; the closed flag does not override it.

Tests

New GetBoundingBoxOfAPeriodicSplineStaysOnTheCurve builds a periodic spline away from the origin and asserts the box stays within the convex hull of the control points — a property every B-spline has, and one the previous behaviour violated. It fails against the current implementation.

dotnet test on this branch: 2314 passed / 17 failed. master at 592d70a on this machine is 2313 passed / 17 failed — the same failures (SingleMLeader, GeoData, InsertWithSpatialFilter), plus the one test this PR adds.

Related

Found while reconciling computed extents with the extents AutoCAD reports for eighteen architectural drawings, alongside #1223 and #1224.