feat: certify minmod mass preservation

[demirelo]

Summary

• prove periodic flux-difference mass preservation for the reconstructed minmod update
• add nonconstant-state and constant-state acceptance witnesses
• include the extension in the theorem inventory and both axiom audits

The theorem assumes only the physical step domain and adds no CFL upper bound.

Verification

• lake env lean -E warning Tests/R5Mass.lean
• lake env lean -E warning Tests/R5AxiomAudit.lean
• lake build --wfail