[wangyinz]

Fixes #870.

Summary

• validate Nsamp and the expected sample interval before scanning or mutating an ensemble
• use the documented half-sample cutoff and pre-scan every live member so abort mode is atomic
• in non-abort mode, kill and log each invalid member exactly once and mark an all-invalid ensemble dead
• normalize the adjacent sort_ensemble fatal exception contract

Verification

• new and existing seismic utility tests: 43 passed
• exact baseline contract run: 26 failed, 10 passed
• independent agent review: PASS, zero blockers
• Black, py_compile, and git diff --check: passed

This PR is ready for human review. It must not be merged automatically.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.29%. Comparing base (431f962) to head (d323533).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #907      +/-   ##
==========================================
+ Coverage   71.28%   71.29%   +0.01%     
==========================================
  Files         180      180              
  Lines       35475    35483       +8     
  Branches     2510     2510              
==========================================
+ Hits        25289    25299      +10     
+ Misses       9286     9284       -2     
  Partials      900      900              

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