potential/backend: centrally coerce forced-backend scalar leaks

[jobovy]

What

Burndown PR-3b — the central scalar-coercion fix, built on #983's galpy.backend._coerce. Under the forced-backend all-backend suite, plain numpy/Python scalars leak into strict torch ops (torch.cos(0.0) / torch.sqrt(numpy.float64) / numpy.ndarray*Tensor all raise; jax is permissive). coerce_coords already handles passed coordinates; this closes the three remaining scalar-leak channels — all guarded so numpy stays byte-identical:

1. conversion.py potential_physical_input: also coerce the phi/t signature defaults (bound via inspect.signature) when omitted, strictly inside the xp is not numpy and _check_backend_compatible guard — so internally-derived scalars (Ferrers _pa+_omegab*t, DehnenBar _smooth(t), CosmphiDisk/HenonHeiles/Logarithmic sin/cos(phi), …) see backend tensors.
2. backend/_coerce.py promote_scalars: use is_backend_array() (not hasattr(v,'ndim')) as the leave-it test, so a numpy.float64 (has .ndim but torch rejects) is promoted.
3. backend/_namespaces.py asarray_on_device: translate a numpy dtype arg to the backend dtype (torch.asarray(dtype=numpy.float64) raises) — fixes SpiralArms.

Plus 19 potentials: move self._backend_compatible=True above self.normalize(...) in __init__ (so construction-time normalize()→Rforce(1.,0.) is coerced); and FerrersPotential axisymmetric phi-reset 0.0 → zeros_like_backend(xp,R).

Impact

Flips ~331 torch test_potential ledger entries to pass (forceAsDeriv / evaluateAndDerivs / 2ndDeriv / poisson / normalize / toVertical_toPlanar, across the potential families). The ledger entries themselves are pruned by the separate ledger-regen PR.

Byte-identity + review

numpy path byte-identical (the whole mechanism is gated on xp is not numpy): verified hash-matched Phi/forces/dens across the 19 reordered potentials + Ferrers + planar default-phi paths; 281/281 numpy parity; all 3325 backend unit tests pass. Independently adversarially reviewed across 4 angles — zero blockers: byte-identity, the inspect.signature default-injection correctness (every signature shape: positional/kwarg/omitted, double-decoration via @wraps, DF-skip, None defaults, no kwonly/posonly), the 19 __init__ reorders, and regression (no previously-passing test breaks; deferred cases honestly still ledgered).

Deferred (separate follow-ups)

• AnySphericalPotential — scipy interior; test_backend_conventions pins it as the canonical unmigrated potential, so it needs a coordinated source+test migration.
• AnyAxisymmetricRazorThinDiskPotential — scipy interior.
• specialSpiralArmsPotential — a torch float64 FD-precision artifact (the crash is fixed; the force matches numpy to 3.5e-8).

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.93%. Comparing base (69659c6) to head (afe3aa9).
⚠️ Report is 1 commits behind head on feat/backends.

Additional details and impacted files

@@               Coverage Diff               @@
##           feat/backends     #987    +/-   ##
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  Coverage          99.93%   99.93%            
===============================================
  Files                254      254            
  Lines              39820    40286   +466     
  Branches             837      835     -2     
===============================================
+ Hits               39795    40261   +466     
  Misses                25       25            

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[github-actions]

All-backend test status (jax / torch)

Commit ade6e60524003e2000dde1287445a86353bb21dd

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: 1056 passed · 266 xfail · 725 deferred | torch: 879 passed · 1164 xfail · 1 deferred

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

Test shard	jax	torch
actionAngle	✅ 112 pass · 89 xfail	✅ 37 pass · 164 xfail
sphericaldf	✅ 164 pass · 26 xfail · 28 deferred	✅ 8 pass · 210 xfail
conversion + util + misc	✅ 85 pass · 6 xfail · 1 deferred	✅ 42 pass · 50 xfail
potential + scf + multipole	— (no result)	— (no result)
quantity + coords	✅ 287 pass · 49 xfail	✅ 206 pass · 130 xfail
orbit (energy/Jacobi + from_name)	✅ 0 pass · 0 xfail · 115 deferred	✅ 65 pass · 50 xfail
orbit + orbits (main)	✅ 0 pass · 0 xfail · 578 deferred	✅ 262 pass · 313 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	✅ 28 pass · 2 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	37	164	0	0	0	0
sphericaldf	jax	164	26	28	0	0	0
sphericaldf	torch	8	210	0	0	0	0
conversion + util + misc	jax	85	6	1	0	0	0
conversion + util + misc	torch	42	50	0	0	0	0
potential + scf + multipole	jax	—	—	—	—	—	—
potential + scf + multipole	torch	—	—	—	—	—	—
quantity + coords	jax	287	49	0	0	0	0
quantity + coords	torch	206	130	0	0	0	0
orbit (energy/Jacobi + from_name)	jax	0	0	115	0	0	0
orbit (energy/Jacobi + from_name)	torch	65	50	0	0	0	0
orbit + orbits (main)	jax	0	0	578	0	0	0
orbit + orbits (main)	torch	262	313	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	28	2	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