Add full-orbit (FO) Boris pusher (orbit_model=7)

[krystophny]

Risk tier

• T3: physics, output behavior, coordinate convention

Correctness contract

Intended behavior change

Adds a new orbit model: orbit_model = 7 (ORBIT_FULL_ORBIT) selects a
gyro-resolved full-orbit (FO) Boris pusher, the ASCOT-style counterpart to the
guiding-center (GC) model. A charged particle advances by explicit Boris
drift-rotate-drift in Cartesian (x, v); field and geometry come from the
production Boozer field through the chartmap (B = curl A, tangent to the flux
surface), with the magnetic axis healed by a pseudo-Cartesian (X,Y,phi) chart.
Output z(1:5) is the guiding-centre reduction each step, so times_lost and
confined_fraction are written exactly as for GC.

Behavior that must not change

orbit_model = 0 (guiding center, the default) is untouched: the symplectic GC
path, its init, and its macrostep are unchanged. No existing test output moves.

Coordinate / unit conventions

Cartesian (x, v) in the scaled chartmap frame; logical chart u=(rho, theta_B,
phi_B) with rho = sqrt(s). CGS-style normalized velocity with the GC sqrt(2)
convention, matching the GC seed so both integrators start from the identical
particle. orbit_coord = 1 (Boozer) is required and enforced.

Numerical invariants

Energy is conserved to machine precision (the Boris rotation is exact for
constant B over a step; no electric field here). The only confinement loss is
the guiding-centre crossing s >= 1; a field-inversion non-convergence is a
numerical fault (fo_fault), reported but never counted as a loss.

Tests added

• unit:
• integration: test_fo_boris (energy bound < 1e-3, near-axis crossing,
  no spurious loss; passing/trapped/near-axis on the reactor-scale Boozer field)
• system:
• golden record:

Golden-record impact

• unchanged

GC is the default and is untouched; FO is opt-in via orbit_model = 7.

Failure modes considered

• Field-inversion non-convergence near a field-period seam: classified as a
  numerical fault (fo_fault), never a loss, so a mid-radius particle is not
  spuriously lost.
• Near-axis singularity of the polar chart: healed by the pseudo-Cartesian
  (X,Y,phi) chart; the near-axis test orbit crosses s in [0.04, 0.07] with
  energy bounded.
• Unsupported combinations (wall loss, collisions, orbit classification, or
  orbit_coord /= 1) stop at config validation with a clear message instead of
  running silently wrong.

Manual validation

Reactor-scale W7-X benchmark (separate run): FO confined fraction agrees with
ASCOT5 full orbit (0.894 vs 0.898 at 1 ms, 1000 alphas), and with GC within
marker statistics.

Verification

Before: there is no full-orbit model on main; orbit_model and the FO modules
do not exist.

$ git grep -l 'ORBIT_FULL_ORBIT\|orbit_fo_boris' origin/main -- src test
(no matches)

After: test_fo_boris passes and the fast suite is green.

$ make test TEST=fo_boris
1/1 Test #6: test_fo_boris ....................   Passed    6.01 sec

  passing   s band [0.5000,0.7700]  max|dE/E0|=2.22E-14  ierr_lost=0
  trapped   s band [0.4999,0.5894]  max|dE/E0|=1.11E-14  ierr_lost=0  drift=8.85E-03
  near-axis s band [0.0400,0.0708]  max|dE/E0|=8.88E-15  ierr_lost=0
  ALL FO-BORIS TESTS PASSED

$ make test-fast
100% tests passed, 0 tests failed out of 53

Relationship to #408

This is the working full-orbit Boris slice, extracted clean off main. The
experimental models on #408 (CPP Pauli, the adaptive RK45 ORBIT_CP6D_RK, the 6D
canonical CP/CPP, device/mock variants) stay on that branch; #408 rebases on top
once this merges.

[krystophny]

Depends on libneo #325 (fix transposed Cartesian Jacobian from chartmap covariant_basis, cart-spline charts). The FO field assembly builds B = curl A through ref_coords%covariant_basis; without that fix on libneo main, a fresh CI build of this branch falls back to libneo main and gets the transposed Jacobian, silently flipping the field. test_fo_boris now asserts the Jacobian convention directly (covariant_basis vs finite-difference of evaluate_cart), so this fails loudly instead of passing with wrong orbits. Merge libneo #325 before merging this.