Varying slopes: multiple slopes per factor (V≥2) + full rank-drop contract (#60)

[schroedk]

Closes #60.

Generalizes the within-level reparametrization to any slope count on the sole factor: one-pass multivariate Welford moments per level, pivoted Gram–Schmidt on the centered slope Gram (raw for slope-only terms) taking the largest-remaining-variance direction each step, and a whitening map whose transpose back-transforms coefficients — dropped directions come back as exact zeros with a data-independent layout.

• Rank-drop contract: within-level collinear slopes drop exactly one direction (the pivot picks the survivor). The relative tolerance is crate-internal (1e-10, per-column so it is scale-invariant); a zero tolerance keeps near-degenerate directions, tested at the unit seam.
• Pivots are tracked through original column indices with no physical swapping, structurally excluding the prototype's ≥3-slope pivot-bookkeeping cliff; a property test forces a non-monotonic pivot order and asserts W·G·Wᵀ = I.
• The gate now rejects only slope terms alongside other terms (Varying slopes: cross-factor signed routing — balance + scale + Schur (balanced/scalable) #61). V=1 behavior is unchanged.
• Tests (bare-minimum set): the two whitening-seam unit tests above, one consolidated V=3 integration test (bounded iterations, exact rank-drop reporting, residual-orthogonality + fitted-reconstruction oracle), and weighted V=3 pyfixest parity.