feat(gam): add derivative-penalized B-splines

[Ananyapam7]

Implements upstream-supported mgcv bs="bs" behavior, including exact integrated-derivative penalty roots, m normalization, all supported knot forms, prediction derivatives and extrapolation, multiple penalties, select/fixed/pc/linked terms, and supported tensor/factor-smooth integrations.

Validation:

• 39 targeted BS parity tests passed from the committed SHA
• 57 linked/factor/penalty regression neighbors passed
• 148/149 broader formula/spec/pipeline tests passed; the sole failure is a pre-existing diagnostics label expectation and reproduces on an isolated clean worktree at the base SHA
• ruff and diff checks passed
• 29/29 new static mgcv fixtures are traced and required

Known unrelated baseline fixture gaps remain for fs_2d_base_tp, sz_2d_base_tp, and sz_2d_base_ts; this bounded PR does not regenerate them.