Returns the $shapes data frame — a tagged uniform schema with one row
per set, a type column, and shape-specific columns
(h, k, a, b, phi, width, height, side) populated according to the
chosen shape (other columns are NA). For circle/ellipse fits the
legacy $ellipses slot is still populated for back-compat; consumers
that need the new shapes (rectangle, square) must read $shapes.
Usage
# S3 method for class 'euler'
coef(object, ...)