evaluate_solovev writes the poloidal field in the opposite orientation family from the rest of the module

[HongSik-Yun-Fusion]

`evaluate_solovev` in `vaft/process/_equilibrium_parametric.py` returns `b_r = -dpsi_dz/R` and `b_z = +dpsi_dr/R`, which is the `k = +1` form, the orientation family with a positive product of the toroidal and poloidal sign conventions (COCOS 1/4/5/8).

Everywhere else in the module the poloidal field comes from `_bp_factor`, whose fallback for an unidentified convention is `k = -1` (COCOS 2/3/6/7). So the analytic Solovev field is in the opposite family from every numerically derived one.

`solovev_to_equilibrium` reconciles the two only on the 2pi axis: its docstring and its code handle the full-weber scaling and say nothing about the sign. A round trip through `solovev_to_equilibrium(convention=11)` into `derive_global_descriptors` gets the factor of 2pi right and, if the declared index is honoured for the sign, can invert the field relative to the analytic `b_r` and `b_z` it came from.

Decide which family the analytic model is in, state it, and make the export reconcile both halves rather than one.

Found while documenting the equilibrium layer under the process documentation contract (#419). Filed rather than fixed, per that issue's rule that a convention applied inconsistently is a numerical-correctness bug with its own review.

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[HongSik-Yun-Fusion]

Resolved via PR #682 (merged into develop).

Summary of Resolution

1. Convention Parameter in evaluate_solovev:
   • evaluate_solovev now accepts cocos: int | None = VAFT_INTERNAL_COCOS (default COCOS 11 / IMAS) as well as a convention: int | None = None alias.
   • The poloidal field orientation factor $\sigma = \sigma_{R\varphi Z} \sigma_{B_p}$ is evaluated dynamically from cocos_spec(cocos):
     $$B_R = +\frac{\sigma}{R} \frac{\partial\psi}{\partial Z}, \quad B_Z = -\frac{\sigma}{R} \frac{\partial\psi}{\partial R}$$
   • When cocos=None is passed, it retains the historical fallback ($\sigma = -1$, COCOS 2/3/6/7 orientation family).
2. Reconciliation in solovev_to_equilibrium:
   • solovev_to_equilibrium now generates a base equilibrium in COCOS 11 (per-radian flux, $\sigma = +1$) and transforms to the target convention using convert_cocos(eq_11, convention) when convention != 11.
   • This reconciles both the $2\pi$ full-weber flux scaling and the orientation sign family consistently.
3. Documentation & Notebook Updates:
   • Updated docstrings in _equilibrium_parametric.py and cell 73 of notebooks/analytic_solovev_equilibrium.ipynb to match COCOS 11 signs.
4. Verification:
   • Added unit tests in test/test_parametric_equilibrium.py covering all 16 COCOS indices, fallback cocos=None, alias convention=..., and convention round-trips.