[FIX] ASR: uncentered scm block covariance + overlap/bound/jump fixes

[sappelhoff]

Fixes a coordinated cluster of issues in block_covariance:

• Uncentered scm covariance: for estimator="scm" compute the uncentered per-block second moment E[x x.T] = B @ B.T / window directly, matching ASR.transform (cov = 1/N * X @ X.T) and the ASR spec (the calibration covariance is not mean-subtracted). pyriemann's scm mean-centers each block, which diverges the calibration matrix M from the correct value on real data and makes calibrate inconsistent with transform. Other estimators still route through pyriemann covariances.
• Loop bound: the loop bound is now computed against the padded length. Previously n_samples was captured before padding, so padding added zero iterations and trailing samples were dropped. A window too large to form any complete block now raises a clear ValueError.
• Overlap convention: jump now uses round(window * (1 - overlap)) so overlap is the fraction of overlap (higher = more overlap), matching clean_windows and asr_calibrate. Previously a larger overlap produced a larger jump (less actual overlap).
• Robustness: jump is clamped to >= 1 so zero-overlap no longer loops forever, and window is cast to int so fractional windows don't break slicing.

Testing

Adds one regression test per fix. Full tests/test_asr.py passes; ruff clean.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.15%. Comparing base (ba7aa78) to head (a3bd391).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #110      +/-   ##
==========================================
+ Coverage   83.11%   83.15%   +0.04%     
==========================================
  Files          25       25              
  Lines        2825     2832       +7     
==========================================
+ Hits         2348     2355       +7     
  Misses        477      477              

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[sappelhoff]

Note: includes the nonlinear_eigenspace reproducibility fix from #118. test_asr_class[*-riemann] compares incremental-vs-bulk output with a loose tolerance and is flaky on CI because that path used an unseeded RNG; this can trip on any PR regardless of what it changes. #118 is the root fix — once it merges first, this commit drops out on rebase.