release: prepare Biosigmat v0.3.1

[dcajal]

Closes #81

Summary

• bump the authoritative Biosigmat version and CFF metadata to 0.3.1 dated 2026-08-26
• publish concise changelog notes for citation metadata, guidance, and release validation
• preserve scientific behavior, references, and the exact Biosiglib pin

Validation

• CFF 1.2.0 validation passed
• MATLAB suite: 256 passed, 0 failed, 0 incomplete
• header validation: 31/31 functions and 17/17 examples compliant
• strict MkDocs build passed
• exact git archive contains a valid CITATION.cff for 0.3.1
• extracted source archive retested: 256/256 passed; docs regenerated and built strictly
• git diff --check passed