potential: backend-correct ttensor eigenvalues + zvc_range no-solution guard

[jobovy]

What

Burndown PR-5 (Potential.py only). Two galpy-source backend fixes that complete the jax test_potential burndown (these were the last 2 jax xfails):

• ttensor (class method + module function): jax.numpy.linalg.eigvals returns a complex array for the symmetric tidal tensor (numpy returns real), breaking the host-side numpy.fabs. Take xp.real(...) on the non-numpy branch only (not eigvalsh — that would change numpy's LAPACK routine/ordering).
• zvc_range backend branch: replicate the numpy path's no-solution [nan,nan] guard via xp.where on g(RLz)>0. The nan branch uses xp.full_like(RLz, nan) (a true constant), not RLz*nan — the latter NaN-poisons d(zvc_range)/dLz through the eager xp.where VJP (caught by adversarial review; FD-verified the gradient is now correct: AD=FD=1.3554).

Byte-identity + review

numpy branch untouched and bit-identical (eigenvalues + zvc results verified via .tobytes()). Fixes jax test_ttensor + test_zvc_range_undefined. Adversarially reviewed (caught + fixed the AD-poisoning blocker; no-solution branch verified crash-free; condition fidelity confirmed). The torch counterparts need a separate test-side wrap / the central coercion and stay ledgered.

🤖 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 (f9f107e).

Additional details and impacted files

@@              Coverage Diff               @@
##           feat/backends     #984   +/-   ##
==============================================
  Coverage          99.93%   99.93%           
==============================================
  Files                253      253           
  Lines              39803    39809    +6     
  Branches             839      840    +1     
==============================================
+ Hits               39778    39784    +6     
  Misses                25       25           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[github-actions]

All-backend test status (jax / torch)

Commit a17b559d98e29fb30b33e31911d3696803e64d71

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: 1057 passed · 265 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	✅ 287 pass · 49 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	287	49	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