orbit: make Orbit.L() cross-product namespace-consistent

[jobovy]

What

Burndown PR-6 (Orbits.py only). Under a forced jax/torch backend, L()'s position/velocity accessors were inconsistent (some returned a backend Tensor, some stayed numpy), so out=numpy.zeros(...); out[...,0]=y*vz-z*vy hit numpy.ndarray * Tensor → TypeError. Resolve one namespace from all six components, coerce onto it only when non-numpy, and assemble via xp.stack(..., axis=-1) (also avoids jax's immutable-array in-place assign).

Byte-identity + review

numpy path byte-identical in data and flags: get_namespace(<all numpy>) is numpy, coercion skipped, and the result is passed through numpy.ascontiguousarray to restore the historical numpy.zeros()+assign C-contiguous layout (xp.stack(axis=-1) of the F-contiguous accessor outputs is otherwise non-C-contiguous for ensemble shapes — caught by adversarial review). Verified SHA-256 identical + C_CONTIGUOUS for single/ensemble/init-time L(). Fixes the torch Orbit.L() TypeError; the other L() code paths (1D/2D/phasedim-5) are untouched.

🤖 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 (07754ee).

Additional details and impacted files

@@              Coverage Diff               @@
##           feat/backends     #985   +/-   ##
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  Coverage          99.93%   99.93%           
==============================================
  Files                253      253           
  Lines              39803    39806    +3     
  Branches             839      836    -3     
==============================================
+ Hits               39778    39781    +3     
  Misses                25       25           

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[github-actions]

All-backend test status (jax / torch)

Commit 8b3429169ce7204aa198a911ae7c42b0aef0d1a0

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: 766 passed · 1277 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	✅ 54 pass · 61 xfail
orbit + orbits (main)	✅ 0 pass · 0 xfail · 578 deferred	✅ 190 pass · 385 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	54	61	0	0	0	0
orbit + orbits (main)	jax	0	0	578	0	0	0
orbit + orbits (main)	torch	190	385	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