js229/MathematicalPhyllotaxis: Second-edition-prerelease-6.61

This release contains the LaTeX and image sources for an uncorrected draft of the second edition of the textbook Mathematical Phyllotaxis.

A limited first printing of the first edition with ISBN 978-0-9931789-6-2 was published by Infang Publishing in September 2023. Compared to that edition, this release incorporates additional material on disk-stacking models from https://arxiv.org/abs/2407.05857 alongside minor corrections and a shift to a landscape format. Please let me have any comments at jonathan@swintons.net.