ledger: jax-slow-skip test_streamspraydf test_center (composite-gate makes its composite eager-jax, >300s)

The composite backend-compat gate (this PR) makes streamspraydf's composite
potential coerce to jax arrays under forced jax; the unmigrated streamspraydf
track-build is eager-jax (no internal jit) and exceeds the 300s pytest-timeout.
Already torch-xfail in the ledger; add the matching jax slow-skip (numpy path
unaffected). Un-skip when streamspraydf/jax is vectorized.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: jobovy

tests/backend_slow_skip.txt

@@ -125,3 +125,4 @@ jax tests/test_potential.py::test_poisson_surfdens_potential[mockRotatedAndTilte
 jax tests/test_potential.py::test_poisson_surfdens_potential[mockRotatedAndTiltedMWP14WrapperPotentialwInclination]
 jax tests/test_potential.py::test_poisson_surfdens_potential[mockRotatedAndTiltedTriaxialLogHaloPotentialwInclination]
 jax tests/test_potential.py::test_poisson_surfdens_potential[mockRotatedTiltedOffsetMWP14WrapperPotential]
+jax tests/test_streamspraydf.py::test_center