Multi-backend (JAX / PyTorch / array-API) support [integration branch]

[jobovy]

Long-lived integration branch for making all of galpy backend-agnostic and differentiable (numpy/scipy, JAX, PyTorch, array-API). This PR establishes the integration branch: the parallel potential-migration series and the autodiff / action-angle / df work all target feat/backends, are reviewed individually as PRs into it, and the whole branch is merged to main at the end (rebased on main after the variational feature branch #891 merges, since differentiable C orbit integration needs its state-transition matrix).

Draft: integration branch, not for merge until the full stack lands.

Foundation (in this PR)

• PR0 — galpy.backend package: a data-first array-namespace resolver (get_namespace: explicit xp= > forced default > array namespace > context/global > numpy), use() context manager, set_default_backend(). numpy/Python inputs resolve to plain numpy (byte-identical path); jax/torch select jax.numpy / array-api-compat torch. Pilot-migrates PlummerPotential + IsochronePotential to private-layer dispatch. 70 new tests green on numpy/jax/torch (parity, grad-vs-FD, the d(Φ)/dR == −Rforce identity, jit/vmap, dtype, decorator pass-through, precedence).
• P1 — productionized: config [backend] section, a force mode (use(..., force=True)) so a whole run can be pinned to a backend, a --backend {numpy|jax|torch} pytest option + autouse fixture (numpy = no-op), and an AST convention guard banning bare numpy.* in migrated compute methods. Verified --backend jax re-runs test_energy_jacobi_conservation (Plummer/Isochrone) green.

Series targeting this branch

• P2.x — potential namespace-swap, parallel fan-out by family (spherical / disk / halo-bar / ellipsoidal / dissipative now in progress; then scipy.special-backed after Pspecial; wrappers last).
• Pspecial — pure-backend special functions (native-preferring router + isolated fallbacks).
• Tracks D/E/F — autodiff wrappers + in-backend integrators, action-angle, df/streams.

Design: galpy_backend_autodiff_discussion.md; project log in the galpy-jax repo.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.92%. Comparing base (27e01fb) to head (d3cccc0).

Additional details and impacted files

@@           Coverage Diff            @@
##             main     #892    +/-   ##
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  Coverage   99.92%   99.92%            
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  Files         225      243    +18     
  Lines       34224    35161   +937     
  Branches      709      709            
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+ Hits        34197    35136   +939     
+ Misses         27       25     -2     

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