fix(rdkit): correct FractionCSP3 / CalcFractionCSP3 casing in docs (fixes #242)

[yunaremaia]

Summary

Fixes #242. PR #213 fixed skills/rdkit/scripts/molecular_properties.py to use Lipinski.FractionCSP3, but reference docs still documented the broken names Descriptors.FractionCsp3 and rdMolDescriptors.CalcFractionCsp3.

Against RDKit 2026.03.x those throw:

Descriptors.FractionCsp3(mol)
# AttributeError: module 'rdkit.Chem.Descriptors' has no attribute 'FractionCsp3'

rdMolDescriptors.CalcFractionCsp3(mol)
# AttributeError: ... Did you mean: 'CalcFractionCSP3'?

This PR updates the documented APIs to the working names:

• Descriptors.FractionCSP3 / Lipinski.FractionCSP3
• rdMolDescriptors.CalcFractionCSP3

Changes:

• skills/rdkit/references/descriptors_reference.md: section title, usage snippet and diversity helper (FractionCsp3 → FractionCSP3)
• skills/rdkit/references/api_reference.md: CalcFractionCsp3 → CalcFractionCSP3
• skills/rdkit/SKILL.md: bump metadata.version 1.2 → 1.3

How tested

uv run --with rdkit python -c "from rdkit import Chem; from rdkit.Chem import Descriptors; from rdkit.Chem import rdMolDescriptors; mol=Chem.MolFromSmiles('CCO'); print(Descriptors.FractionCSP3(mol)); print(rdMolDescriptors.CalcFractionCSP3(mol))"
# 1.0 / 1.0

uv run skills-ref validate ./skills/rdkit
# Valid skill: skills/rdkit

uv run --with pytest python -m pytest tests/_meta -q
# 10 passed, 1213 subtests passed

Verified the old names are absent (grep -rn FractionCsp3) and no longer exist in the installed RDKit.

Fixes #242.