fix(rdkit): finish FractionCSP3 / CalcFractionCSP3 doc corrections

[Divyansh151005]

Summary

PR #213 fixed molecular_properties.py to call Lipinski.FractionCSP3, but the reference docs still taught the old names that crash on RDKit 2026.03.x.

• descriptors_reference.md: Descriptors.FractionCsp3 → Descriptors.FractionCSP3 (section + diversity example)
• api_reference.md: rdMolDescriptors.CalcFractionCsp3 → CalcFractionCSP3
• Bump metadata.version "1.2" → "1.3"

Type of change

• Update to an existing skill

Skills touched

• rdkit

How this was tested

$ uv run --with rdkit python - <<'PY'
from rdkit import Chem
from rdkit.Chem import Descriptors, Lipinski, rdMolDescriptors
m = Chem.MolFromSmiles('CC(=O)Oc1ccccc1C(=O)O')
Descriptors.FractionCsp3(m)          # AttributeError
Descriptors.FractionCSP3(m)          # 0.111...
Lipinski.FractionCSP3(m)             # 0.111...
rdMolDescriptors.CalcFractionCsp3(m) # AttributeError
rdMolDescriptors.CalcFractionCSP3(m) # 0.111...
PY

$ 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

Against RDKit 2026.03.5.

Related issues and references

Fixes #242. Completes the doc half of #213 / #97.

Checklist

Skill format

• The skill directory name and the name frontmatter match exactly.
• The skill directory contains only SKILL.md, references/, scripts/, and assets/ — no tests/ directory and no test_*.py files.
• SKILL.md has valid YAML frontmatter and a Markdown body.
• Only the six spec-defined top-level fields are present; everything else lives under metadata.
• metadata is a block mapping, not single-line JSON, and scalar values are quoted where needed.
• metadata.version exists, is quoted, and is bumped if an existing skill changed.
• The description says both what the skill does and when an agent should use it.

Validation and tests

• uv run skills-ref validate ./skills/rdkit passes.
• Relevant test suites pass (tests/_meta).
• Security scanner results are clean or explained in this PR. — docs-only; no scripts/ change, scanner not run.

Content and safety

• Examples and scripts were tested, or are clearly marked as illustrative.
• No secrets, credentials, private data, or unsafe instructions are included.

Notes for reviewers

Left MolToSmarts(..., isomericSmiles=False) default documentation alone (same deliberate scope boundary as #213).