nx_chem_valence.nx

nx_chem_valence.nx -- C2.3a milestone: implicit-H valence inference + molecular weight computation. Provides: nx_chem_default_valence(z) -- Daylight 1988 default valence (organic subset) nx_chem_compute_implicit_h(m) -- walks atoms; sets h_count for -1 sentinels per (valence - bond_order_sum) for unambiguous cases nx_chem_mol_weight_q3(m, table) -- sum atomic weights * 1000 (milli-AMU) including implicit Hs; matches NIST WebBook standard molecular weight Honest gaps (preserved for future milestones): - Aromatic non-carbon atoms (n / o / s / p in rings) leave h_count = -1 because pyrrole- vs-pyridine disambiguation requires C2.5 ring-electron-counting; substrate refuses to fabricate H counts rather than guess wrong. - Hypervalent forms (N+5, S+4/+6, P+5, etc.) require explicit bracket specification. - Non-organic-subset atoms (Si, B with non-standard, transition metals, etc.) leave h_count = -1 unless specified in brackets. nx_safety_envelope: intended_use: implicit-H + molecular-weight for cheminformatics + MS downstream sil_target: SIL1 evidence: [C2.3a KAT in nx_chem_valence_test.nx; NIST WebBook reference values] verdict: BENCH-PENDING