[Quality frontier] Complete evidence-based repository hardening

[edithatogo]

Refined audit snapshot

Default branch: master; kind: software; workflows: 6.

Technique	Present	Warranted
Property-based	yes	yes
Mutation	yes	yes
DST	no	no
MT	no	yes
Contract	no	yes

Subissues

• [Quality frontier] Close warranted testing and CI/CD gaps #161
• [Quality frontier] Maximise security and solo-maintainer context #162

Close only when every subissue is complete or records an evidence-backed exclusion. Keep the repository operable by one maintainer.