Use xp.stack in messages so JAX can trace fixed arrays - #1460

Summary

• replace ten fixed-shape xp.array([...]) message constructors with xp.stack([...])
• dispatch GammaMessage.to_canonical_form through xp.log instead of NumPy
• add direct scalar and batched jax.jit coverage for every changed constructor, with NumPy shape/value parity

Why

The Gamma canonical-form path currently sends a traced value through np.log, raising TracerArrayConversionError. Using the selected array namespace throughout keeps message sufficient-statistic construction traceable and makes the fixed-array construction consistent across NumPy and JAX.

The same regression test was run against untouched main: the scalar and batched Gamma canonical-form cases fail there and pass on this branch. The other array constructors already pass with current supported JAX, so their xp.stack conversion is cross-backend hardening with unchanged shapes and values.

Verification

• test_autofit/messages: 32 passed
• full test_autofit: 1689 passed, 4 skipped
• baseline proof: 2 expected failures on untouched main, both GammaMessage.to_canonical_form; 20 passed on this branch

The private z_projects/concr integration reproducer was not available in this environment.

Prior/message audit

The adjacent audit found four separate JIT backend leaks in Gamma/Beta log_partition and compound-prior log/log10. They are independently reproduced and tracked in #1459 rather than widening this PR.

Closes #1458