Fix HEALPix spin!=0 inverse/forward inaccuracy from Wigner-d node

[slosar]

Note: this bug was uncovered and fixed by agentic AI tools, but it is a real non-trivial bug whose fix should be merged. We need sasfe_abs for avoit hitting nans for auto-diff. There is a test added that fails in the current main version, but passes here, so it would be good to keep it. Below is the machine generated description:

The on-the-fly Price-McEwen Wigner-d recursion renormalises each step by bigi = 1/|dl_entry| and tracks lbig = log|dl_entry|. When an intermediate recursion value dl_entry is exactly zero (a node), bigi becomes inf and lbig becomes -inf, so dl_iter = inf*0 = NaN and the running log-norm goes to -inf. These NaNs are silently dropped by the nansum that accumulates ftm/flm, discarding that mode's contribution and producing percent-level errors.

Exact-zero nodes are essentially never hit by the generic theta samples of mw/mwss/dh/gl sampling, but HEALPix rings sit at rational cos(theta) values that land exactly on nodes for spin != 0, so spin-2 HEALPix transforms carried few-percent pointwise errors (spin-0 was unaffected and existing HEALPix tests only covered spin 0).

Guard the renormalisation so that where dl_entry == 0 we use bigi = 1 and lbig = 0, leaving lrenorm unchanged and setting the normalised value to 0, which is exactly consistent with the recursion bookkeeping. Applied to all four recursion paths (numpy/jax inverse and forward).

Adds a regression test comparing the recursive HEALPix spin-1/2 inverse against the independent Turok-recursion base transform.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.11%. Comparing base (4ef1ee5) to head (cefdf46).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #387      +/-   ##
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+ Coverage   96.10%   96.11%   +0.01%     
==========================================
  Files          34       34              
  Lines        3539     3551      +12     
==========================================
+ Hits         3401     3413      +12     
  Misses        138      138              

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