v0.1.8

codebase-review hardening: data-integrity fixes, covariance-contract unification, dead-code cleanup

• Mode-agnostic fit-quality metric nrmse surfaced in nlsq_diagnostics["fit_quality"] / nlsq_result.json.
• Homodyne and heterodyne uncertainties now follow one covariance contract on every solve path — placeholder/singular covariance reports NaN with covariance_is_placeholder instead of a fabricated zero, identity, or floored value.
• Data-integrity fixes: APS-U bin-skip index mislabeling, NPZ cache keyed to its source HDF5 file, homodyne angle-stratified fits no longer fit the c2 diagonal.
• ~4K lines of dead code removed (performance_engine.py, memory_manager dead methods, unreachable ImportError fallback shims) with no numerical or CLI-facing behavior change.

Full details in CHANGELOG.md.