potential: pin SCF/Multipole construction-time setup to numpy

[jobovy]

What

Burndown PR-4 (SCFPotential.py + MultipoleExpansionPotential.py). Constructing SCF/Multipole/DiskSCF/DiskMultipole/KuijkenDubinski potentials ran their pure-numpy coefficient/density-grid quadrature + density-lambda arity autodetect while the forced backend default was torch/jax, so get_namespace(<plain scalar>) returned the backend and torch rejected the mixed ops (numpy.ndarray * Tensor; a swallowed TypeError mis-detecting a Multipole density lambda's arity). Wrap the construction-time numerical setup in with use("numpy", force=True):. This is non-differentiable host setup that builds the stored numpy coefficient tables; evaluation stays backend-aware (the jax/torch eval tables are built lazily, outside the wrap).

Byte-identity + review

numpy byte-identical (forced-numpy is a no-op on the numpy default): 118 stored coefficient/table attributes bit-identical parent vs branch; test_scf/test_MultipoleExpansionPotential/test_amp_mult_divide counts unchanged. Under torch these families now construct and 12 test_amp_mult_divide entries flip to pass; the 3 remaining (expwhole*/nonaxiDiskSCF) fail for the separate Potential.normalize-stores-Tensor central-coercion reason (PR-3b). Adversarially reviewed (byte-identity, eval-path-not-pinned, context-manager exception/nesting safety, no missed/double-wrapped paths — all refuted).

🤖 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 (d02be4c).

Additional details and impacted files

@@              Coverage Diff               @@
##           feat/backends     #986   +/-   ##
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  Coverage          99.93%   99.93%           
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  Files                253      253           
  Lines              39803    39810    +7     
  Branches             839      845    +6     
==============================================
+ Hits               39778    39785    +7     
  Misses                25       25           

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[github-actions]

All-backend test status (jax / torch)

Commit 5125d1aa871db4f2c37bfa3d5334c50c7d697a75

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: 1050 passed · 272 xfail · 725 deferred | torch: 782 passed · 1261 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	✅ 85 pass · 6 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	✅ 62 pass · 53 xfail
orbit + orbits (main)	✅ 0 pass · 0 xfail · 578 deferred	✅ 198 pass · 377 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	85	6	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	62	53	0	0	0	0
orbit + orbits (main)	jax	0	0	578	0	0	0
orbit + orbits (main)	torch	198	377	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