## Summary

Add the full GVEC and real-Cartesian Boozer chartmap layer on top of the
VMEC validation stack.

This PR now absorbs the old top PR 327. The real-Cartesian contract
belongs here, because this is the first layer where the generalized
non-cylindrical Boozer toroidal parameter actually matters end to end.

This layer adds:

- GVEC state -> Boozer chartmap NetCDF conversion
- real-Cartesian `x,y,z` export for the Boozer grid instead of the
earlier pseudo-Cartesian embedding
- `zeta_convention='boozer'` fixtures and refreshed QA/figure-8
chartmaps on that contract
- QA common-case VMEC/GVEC comparison on the shared chartmap path
- figure-8 QUASR/GVEC golden validation and reviewer plots
- benchmark/artifact helpers used by the end-to-end GVEC coverage
- CI installation of `gvec` for the slow GVEC-backed Python tests

## Stack

- base: `feature/boozer-chartmap-qa`
- head: `feature/boozer-chartmap-gvec`

## Validation scope in this PR

- shared QA case across:
  1. direct VMEC Boozer
  2. VMEC-exported Boozer chartmap
  3. GVEC-exported Boozer chartmap
- figure-8 QUASR -> GVEC -> Boozer chartmap -> SIMPLE golden validation

## Review

Primary GVEC/QUASR review: @Rykath
SIMPLE-maintainer follow-up: @krystophny