Add a curve-simplification stage (--simplify), paper.js style

A new pipeline slot between curve fitting and composition: CurvePasses
rewrite each fitted contour, so mosaic mode transforms every shared
boundary segment exactly once and the tessellation stays seam-free by
construction. SimplifyCurves re-fits each smooth run between corners
with the fewest cubics within the tolerance (Schneider's algorithm via
a current flo_curves — visioncortex's copy is pinned to an old one and
block-splits at 200 points), with tangents from the chain's own ends,
corners kept in place, junction endpoints pinned bit-for-bit, and rings
seamed at their sharpest junction. Off by default; polylines pass
through untouched. Cityscape at tolerance 1: 229 -> 138 KB stacked,
103 -> 36 KB watershed cutout, with render diffs under golden noise.

CurveFitter now returns Vec<FittedGeom> (promoted from mosaic::fit) so
stacked contours flow through the same pass machinery; the optimizer's
SimplifyPass is renamed CleanupPass to free the word.

crates/vtracer-py/src/lib.rs

@@ -188,6 +188,8 @@ impl PyConfig {
                 length_threshold,
                 max_iterations,
                 splice_threshold,
+                // Curve simplification is not surfaced in the Python API yet.
+                simplify: None,
                 path_precision: Some(path_precision),
                 palette,
                 max_colors,