feat(gam): add cyclic P-spline smooths

[Ananyapam7]

Summary

• port vendored mgcv 1.9-4 cyclic P-spline knot, wrapped basis, circular difference-penalty, prediction, and derivative behavior
• integrate bs="cp" through formula/array APIs, linked ids, point constraints, by variables, selection/fixed terms, tensor marginals, and fs/sz bases
• add static raw-constructor, smoothCon, fit/inference, boundary, diagnostics, persistence, and combination parity fixtures

Upstream parity source

• R/smooth.r: cwrap, cSplineDes, smooth.construct.cp.smooth.spec, Predict.matrix.cpspline.smooth, plus generic smoothCon/tensor/factor-smooth paths

Verification

• 33 CP-focused static parity tests passed
• 15 legacy PS smoothCon tests and 11 raw PS/factor-base cases passed
• 61 PS-related pc/id/fixed-SP/lifecycle cases passed
• 68 spec-owner, array-persistence, and formula-parse tests passed
• Ruff and git diff --check passed
• full raw/smoothCon run: 140 passed; 3 pre-existing 2D fs/sz cases remain blocked only by missing committed fixtures (55e9a0a, 7b27252, 70af8bb)

Scope

This is the cyclic P-spline (cp) Wave 2 subwave. Remaining Wave 2 basis families stay separate.