backend: centralize data-coercion helpers into galpy.backend._coerce

[jobovy]

What

Burndown PR-3a (foundational refactor). Relocates the backend data-coercion helpers — which were scattered across the wrong modules (util/conversion.py = units, util/coords.py = coordinate systems, two potential files) — into a new galpy/backend/_coerce.py with a guiding top-level docstring (the canonical reference for how backend coercion works + the numpy-passthrough byte-identity invariant). Resolves the long-standing review note to centralize these.

Moved verbatim (renames only):

• coerce_coords (from backend/_namespaces.py)
• promote_scalars (was coords.py::_promote_scalars_for)
• as_backend_constant (was RotateAndTiltWrapperPotential::_as_xp_constant)
• zeros_like_backend (was KuzminLikeWrapperPotential::_zero_like)

Call sites (coords.py, conversion.py, RotateAndTilt/KuzminLike/OblateStaeckel wrappers, two test_backend_* modules) now delegate to galpy.backend; _namespaces.py keeps the namespace/dtype/device primitives; __init__ re-exports the coercion API.

Pure relocation — zero behavior change

Each function body is moved byte-for-byte. No new functionality (the new scalar-default coercion is the follow-up PR-3b).

Byte-identity + review

numpy path byte-identical (verified: test_coords 97/97, test_backend_conventions+test_backend_device 47+1, RotateAndTilt/KuzminLike/OblateStaeckel/Spherical 210/210 — exact parent match; the 4 moved bodies diff to renames only). No circular import. Independently adversarially reviewed (all 6 angles refuted: verbatim-move, all-call-sites-rewired, numpy byte-identity, no-cycle, docstring accuracy, no-backend-drift).

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.93%. Comparing base (fcad3f7) to head (fb796af).

Additional details and impacted files

@@              Coverage Diff               @@
##           feat/backends     #983   +/-   ##
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  Coverage          99.93%   99.93%           
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  Files                253      254    +1     
  Lines              39803    39804    +1     
  Branches             839      836    -3     
==============================================
+ Hits               39778    39779    +1     
  Misses                25       25           

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[github-actions]

All-backend test status (jax / torch)

Commit 3b4bfec9bce8f401526add01c2eda5058519881f

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: 1051 passed · 271 xfail · 725 deferred | torch: 764 passed · 1279 xfail · 1 deferred

Ledger size: 2357 entries (jax=284, torch=2073).

Test shard	jax	torch
actionAngle	✅ 112 pass · 89 xfail	✅ 26 pass · 175 xfail
sphericaldf	✅ 164 pass · 26 xfail · 28 deferred	✅ 8 pass · 210 xfail
conversion + util + misc	✅ 86 pass · 5 xfail · 1 deferred	✅ 41 pass · 51 xfail
potential + scf + multipole	— (no result)	— (no result)
quantity + coords	✅ 281 pass · 55 xfail	✅ 189 pass · 147 xfail
orbit (energy/Jacobi + from_name)	✅ 0 pass · 0 xfail · 115 deferred	✅ 53 pass · 62 xfail
orbit + orbits (main)	✅ 0 pass · 0 xfail · 578 deferred	✅ 189 pass · 386 xfail
evolveddiskdf	✅ 35 pass · 0 xfail	✅ 32 pass · 3 xfail
jeans + dynamfric	✅ 17 pass · 2 xfail · 2 deferred	✅ 7 pass · 13 xfail · 1 deferred
qdf + pv2qdf + streamgapdf_impulse + noninertial	✅ 57 pass · 75 xfail · 1 deferred	✅ 14 pass · 119 xfail
streamgapdf	✅ 28 pass · 2 xfail	✅ 27 pass · 3 xfail
diskdf	✅ 129 pass · 0 xfail	✅ 112 pass · 17 xfail
streamdf + streamspraydf + streamTrack	✅ 142 pass · 17 xfail	✅ 66 pass · 93 xfail

Per-shard counts

Test shard	backend	pass	xfail	deferred	XPASS	fail	error
actionAngle	jax	112	89	0	0	0	0
actionAngle	torch	26	175	0	0	0	0
sphericaldf	jax	164	26	28	0	0	0
sphericaldf	torch	8	210	0	0	0	0
conversion + util + misc	jax	86	5	1	0	0	0
conversion + util + misc	torch	41	51	0	0	0	0
potential + scf + multipole	jax	—	—	—	—	—	—
potential + scf + multipole	torch	—	—	—	—	—	—
quantity + coords	jax	281	55	0	0	0	0
quantity + coords	torch	189	147	0	0	0	0
orbit (energy/Jacobi + from_name)	jax	0	0	115	0	0	0
orbit (energy/Jacobi + from_name)	torch	53	62	0	0	0	0
orbit + orbits (main)	jax	0	0	578	0	0	0
orbit + orbits (main)	torch	189	386	0	0	0	0
evolveddiskdf	jax	35	0	0	0	0	0
evolveddiskdf	torch	32	3	0	0	0	0
jeans + dynamfric	jax	17	2	2	0	0	0
jeans + dynamfric	torch	7	13	1	0	0	0
qdf + pv2qdf + streamgapdf_impulse + noninertial	jax	57	75	1	0	0	0
qdf + pv2qdf + streamgapdf_impulse + noninertial	torch	14	119	0	0	0	0
streamgapdf	jax	28	2	0	0	0	0
streamgapdf	torch	27	3	0	0	0	0
diskdf	jax	129	0	0	0	0	0
diskdf	torch	112	17	0	0	0	0
streamdf + streamspraydf + streamTrack	jax	142	17	0	0	0	0
streamdf + streamspraydf + streamTrack	torch	66	93	0	0	0	0