feat: add corner radius support for rect, frame, polygon, and star sh…

…apes (v0.8)

Rectangles and frames read per-corner radii (topLeft/Right, bottomLeft/Right)
from RectangleCornerMixin. Polygons and stars read uniform cornerRadius from
CornerMixin and compute the rounding-circle center per convex vertex to
exclude points in corner-cut regions.

Co-Authored-By: Claude Opus 4.7 <[email protected]>

v0.6 — rotation modes, spiral curl fields, curl modifier

v0.5 — bitmap lightness sampling mode

v0.4: Five grid tiling modes (quad, hex, brick, diamond, random)

v0.3: Gradient color sampling toggle

v0.2: Fixed gradient sampling, default black dots, README

v0.1: Grid Dots plugin initial release