Add span methods to PdfFunction, Shading and ColorSpaceDetails

[BobLd]

Introduces allocation-free Eval(ReadOnlySpan<double>, Span<double>) on PdfFunction and Shading plus GetRgb(ReadOnlySpan<double>, out r, out g, out b) on ColorSpaceDetails; the legacy double[] Eval / IColor GetColor surface is preserved by routing through a virtual wrapper sized by a new MaxOutputComponentCount.

PdfFunctions now caches the numeric arrays once at construction.

Type 0's RInterpol becomes a readonly ref struct that writes each recursion level into a caller-supplied slice of a single scratch span, so the sample interpolation no longer allocates a new double[] at every depth.

ColorSpaceDetails fans GetRgb out across Device / Cal / Lab / Indexed / Separation / DeviceN / ICCBased, with a shared GetRgbViaTint helper for the alternate-space hop, and the Cal/Lab Span Transform overrides now hoist their per-pixel stackalloc out of the loop.