[rogeriojorge]

Adds an exact segment remap (u, a) -> (u', a') per species and axis (Pagliantini et al., arXiv 2208.14373, Def. 1).

• remap_matrix builds T[m,n] (h_m(A xi + B) on h_n(xi), with A = a/a', B = (u-u')/a') by recurrence in O(N^2). It is exactly lower-triangular with T_nn = A^n.
• remap applies T separably with the (a/a')^3 normalization. It is pure and differentiable.
• moment_target gives the moment-matched basis: U = u + a g1/(sqrt2 g0), a' = sqrt2 sigma.
• cap_target clips the target: |du| <= a', a/a' <= 1.1, a' >= 1.1 sigma. Widening is free.
• remap_event refuses moves outside the caps. It records the time, both bases, per-species tail fractions and the low-moment defect.

Why the caps: conditioning grows like (a/a')^N when the basis narrows, and like (du/a)^N/sqrt(N!) with the shift. With measured cond tables, a coherent displacement d >= 3 cannot be remapped at N >= 32. Large coherent shifts belong to the pump frame (PR 2/5).

Tested SHA 13405b8. Command: python -m pytest -q -n 2 tests. Result: 170 passed.

New tests (tests/test_remap.py):

• T agrees with Gauss-Hermite quadrature to 1e-13 for N up to 40; it is lower-triangular with diagonal A^n.
• Every mixed moment v_x^i v_y^j v_z^k with i < Nn, j < Nm, k < Np is preserved to 1e-12 against quadrature (unequal orders, 2 species).
• The round trip is the identity to 1e-13.
• The generator d/ds T(u + u_dot s, a + a_dot s) at s = 0 equals basis_rate_terms to 1e-12.
• Shifted and heated Maxwellians: the target is recovered and the remap gives delta_n0 to 1e-13.
• Caps are enforced and clipped.
• Landau k lambda_D = 0.5, Nn = 128: a run remapped at t = 5 to (0.3 a, 1.1 a) tracks the fixed-basis E_k to 5e-7 relative. Both fit gamma = -0.1537 (reference -0.1534).

Stack: base adaptive/base = integration/dark-baseline 9d0982d (main ab87385 + #48, #46, #13, #12, #9, #55, #50, #44, #18, #51). This lands on main after those foundation PRs and the earlier PRs of this stack (#56 first). Local results only (macOS, Python 3.12, JAX 0.11.2, x64); GitHub CI on this branch has not been checked yet.