Test-suite design pass: gaps, boundary/equivalence, mutation testing

[wimaan3]

Context

Strengthen the suite via black-box design + oracle strength, not just line coverage.

Acceptance criteria

• Identify branch-coverage gaps in core (organize/reconcile/planner/vault).
• Add boundary/equivalence/error inputs with accept-and-reject pairs.
• Mutation testing on core; raise surviving-mutant kills.
• Strengthen weak assertions; remove brittle implementation-coupled tests.

Notes

Coverage is a map not a grade; confidence comes from assertions.