v0.4.0

Removed

• gyro_noise.py — its pre/post-filter gyro PSD is reproduced by the chirp_analysis.py HTML report and its peak/harmonic diagnosis overlaps spectral_analysis.py. Not part of the skill workflow.
• Two unused French reference notes (one orphaned, one only cited from a code comment) — against the English-only references rule.

Added

• scripts/blackbox_signal.py — shared decode/load/sample-rate/activity helpers for the analysers, plus an in-process .bbl→DataFrame decode (no subprocess, no temp file, no CSV round-trip), with the analyze_blackbox --csv subprocess kept as an isolated fallback.
• selftest: synthetic-signal smoke for spectral_analysis + step_response, and an in-process-vs-subprocess decode equivalence guard.

Changed

• Deduplicated the helpers copy-pasted across spectral_analysis, step_response and chirp_analysis (no behaviour change; diverging DSP primitives stay per-script).
• SKILL.md token diet: trimmed the always-loaded description (~132 → ~105 words, trigger coverage kept) and compressed the inline chirp paragraph into a pointer to references/chirp-tuning.md.
• Dropped all external-tool brand mentions; diagnosis conventions phrased generically.