[FIX] ASR: apply online recency weights on the euclid path (weighted geometric median)

[sappelhoff]

Two coupled fixes to the online covariance weighting in ASR.transform():

• The euclid path discarded its recency weights. transform() builds an exponential recency weight vector and passes it to asr_process(sample_weight=...), but on the default method="euclid" path asr_process called geometric_median() with no weights (and the function had no weight parameter), so the weights were computed and silently dropped — only the riemann branch ever used them. Generalize geometric_median() to an optional sample_weight using the weighted Vardi-Zhang iteration; with sample_weight=None (or all ones) it reduces exactly to the previous unweighted algorithm, so asr_calibrate (which calls it unweighted) is numerically unchanged. Pass sample_weight through the euclid branch of asr_process.
• The recency weight vector used prefix sums instead of suffix sums. A block's recency is the number of more-recent samples that follow it, i.e. the suffix sum of the per-block sample counts; the previous prefix-sum index mis-weighted interior blocks whenever chunk sizes differed. Add a _recency_weights helper that accumulates the suffix sample count newest→oldest.

Testing

Adds regression tests for the weighted median (uniform == unweighted), the euclid wiring, and the suffix-sum weights. Full tests/test_asr.py passes; ruff clean.

[codecov]

Codecov Report

❌ Patch coverage is 90.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.22%. Comparing base (8e53091) to head (6d0ab3a).

Files with missing lines	Patch %	Lines
meegkit/utils/asr.py	80.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #112      +/-   ##
==========================================
+ Coverage   83.16%   83.22%   +0.05%     
==========================================
  Files          25       25              
  Lines        2834     2843       +9     
==========================================
+ Hits         2357     2366       +9     
  Misses        477      477              

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[sappelhoff]

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Note: includes the nonlinear_eigenspace reproducibility fix from #118 (needed for the riemann path to have stable CI). Once #118 merges, this commit will drop out on rebase.