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

❌ Patch coverage is 98.72925% with 62 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.98%. Comparing base (7a31c19) to head (de36617).
⚠️ Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
galpy/orbit/orbit_c_ext/integrateFullOrbit.c	74.01%	33 Missing ⚠️
galpy/df/sphericaldf.py	88.34%	19 Missing ⚠️
galpy/orbit/integrateFullOrbit.py	72.22%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #892      +/-   ##
==========================================
- Coverage   99.93%   98.98%   -0.96%     
==========================================
  Files         225      258      +33     
  Lines       37975    42938    +4963     
  Branches      839      899      +60     
==========================================
+ Hits        37951    42501    +4550     
- Misses         24      437     +413     

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[github-actions]

All-backend test status (jax / torch)

Commit 03b7939cb862214af33a96c8ed1d8792b4842410

Green is achieved via the checked-in xfail-ledger (tests/backend_xfail.txt, applied xfail(strict=False)), so the metric to watch is the shrinking xfail count (burndown), not a raw pass count. A FAIL/ERR is an un-ledgered regression (reds the run). Because the ledger is non-strict, a now-passing ledgered test is a plain pass here (no per-push XPASS); burndown candidates -- in both directions -- are surfaced by the scheduled regen run, which rewrites the ledger from real outcomes. deferred is a separate burndown: tests skipped because they are unrunnable under the backend until the port is vectorized (see tests/backend_slow_skip.txt), e.g. the jax spherical-DF sampling/quadrature tests pending the Track F DF migration.

Overall: jax: 2947 passed · 233 xfail · 263 deferred · 10 exempt | torch: 2548 passed · 885 xfail · 14 deferred · 10 exempt

Ledger size: 1119 entries (jax=233, torch=886).

Test shard	jax	torch
actionAngle	✅ 137 pass · 68 xfail · 2 deferred	✅ 92 pass · 115 xfail
sphericaldf	✅ 164 pass · 26 xfail · 28 deferred	✅ 45 pass · 173 xfail
conversion + util + misc	✅ 82 pass · 5 xfail · 5 deferred	✅ 64 pass · 28 xfail
SpiralArms + scf + multipole	✅ 190 pass · 5 xfail	✅ 162 pass · 33 xfail
potential (5 split-groups)	✅ 1192 pass · 0 xfail · 16 deferred	✅ 1118 pass · 94 xfail
quantity + coords	✅ 301 pass · 35 xfail	✅ 244 pass · 92 xfail
orbit (energy/Jacobi + from_name)	✅ 98 pass · 0 xfail · 13 deferred · 4 exempt	✅ 94 pass · 17 xfail · 4 exempt
orbit + orbits (main)	✅ 441 pass · 14 xfail · 114 deferred · 6 exempt	✅ 436 pass · 120 xfail · 13 deferred · 6 exempt
evolveddiskdf	✅ 35 pass · 0 xfail	✅ 35 pass · 0 xfail
jeans + dynamfric	✅ 17 pass · 2 xfail · 2 deferred	✅ 12 pass · 8 xfail · 1 deferred
qdf + pv2qdf + streamgapdf_impulse + noninertial	✅ 57 pass · 75 xfail · 1 deferred	✅ 17 pass · 116 xfail
streamgapdf	✅ 30 pass · 0 xfail	✅ 28 pass · 2 xfail
diskdf	✅ 121 pass · 3 xfail · 5 deferred	✅ 129 pass · 0 xfail
streamdf + streamspraydf + streamTrack	✅ 82 pass · 0 xfail · 77 deferred	✅ 72 pass · 87 xfail

Per-shard counts

Test shard	backend	pass	xfail	deferred	XPASS	fail	error
actionAngle	jax	137	68	2	0	0	0
actionAngle	torch	92	115	0	0	0	0
sphericaldf	jax	164	26	28	0	0	0
sphericaldf	torch	45	173	0	0	0	0
conversion + util + misc	jax	82	5	5	0	0	0
conversion + util + misc	torch	64	28	0	0	0	0
SpiralArms + scf + multipole	jax	190	5	0	0	0	0
SpiralArms + scf + multipole	torch	162	33	0	0	0	0
potential (5 split-groups)	jax	1192	0	16	0	0	0
potential (5 split-groups)	torch	1118	94	0	0	0	0
quantity + coords	jax	301	35	0	0	0	0
quantity + coords	torch	244	92	0	0	0	0
orbit (energy/Jacobi + from_name)	jax	98	0	13	0	0	0
orbit (energy/Jacobi + from_name)	torch	94	17	0	0	0	0
orbit + orbits (main)	jax	441	14	114	0	0	0
orbit + orbits (main)	torch	436	120	13	0	0	0
evolveddiskdf	jax	35	0	0	0	0	0
evolveddiskdf	torch	35	0	0	0	0	0
jeans + dynamfric	jax	17	2	2	0	0	0
jeans + dynamfric	torch	12	8	1	0	0	0
qdf + pv2qdf + streamgapdf_impulse + noninertial	jax	57	75	1	0	0	0
qdf + pv2qdf + streamgapdf_impulse + noninertial	torch	17	116	0	0	0	0
streamgapdf	jax	30	0	0	0	0	0
streamgapdf	torch	28	2	0	0	0	0
diskdf	jax	121	3	5	0	0	0
diskdf	torch	129	0	0	0	0	0
streamdf + streamspraydf + streamTrack	jax	82	0	77	0	0	0
streamdf + streamspraydf + streamTrack	torch	72	87	0	0	0	0