Infer derived matrix tactic certificates

[PerAlexandersson]

Summary

• add bare inferred forms for the six derived matrix and row-threshold endpoints
• reuse registered rectangularity, nonnegativity/threshold, and 2x2 certificates
• preserve explicit named and positional frontend coverage and add a decoy-matrix smoke example

Verification

• lake ... build RealRooted.Tactic.Examples.Matrix (8625 jobs)
• lake ... build RealRooted (8951 jobs)
• python3 scripts/check_root_imports.py
• python3 scripts/generate-oeis-tactic-coverage.py --check
• git diff --check
• touched line-length and placeholder scans

All tactics invoke existing proved theorem declarations; no theorem files are changed.