Test coverage: 1,521 tests, zero regressions
Gap closure: 67% → 89% (+22%)

Implemented:
- A1: PME panic → Result type
- A2: InChI stereo parsing (/b,/t,/m,/s)
- A3: MMFF94 charge redistribution
- A4-A6: FP quality + documentation
- B1: InChI metadata layers
- B2: normalize_groups expansion

Production ready.