Add CXSMILES string slot

## Summary Fixes #64. This adds a precise CHEMROF slot for CXSMILES using `cxsmiles_string` as the primary name while preserving the older `extended_smiles_string` slot as deprecated metadata. Changes in this PR: - add `cxsmiles_string` for CXSMILES values - expose `cxsmiles_string` on `ChemicalEntity` for concrete chemical records - expose `cxsmiles_string` on `ChemicalGroupingClass` for generalized/R-group structures like the example in issue #64 - keep `extended_smiles_string` as a deprecated slot and declare `cxsmiles_string` as its exact replacement with `deprecated_element_has_exact_replacement` - document CXSMILES with aliases, Wikidata `wd:P10718`, and the ChemAxon format reference - add the issue #64 R1/R2 CXSMILES example as a valid `MoleculeGroupingClass` example ## Rationale The previous primary name, `extended_smiles_string`, was too broad: several tools and conventions could reasonably be described as extended SMILES. The issue is specifically about CXSMILES, so this PR uses `cxsmiles_string` as the schema-facing name while retaining the previous slot as deprecated for compatibility. The example in the issue uses R-group placeholders, so it is modeled as a grouping class rather than a concrete fully specified molecule. ## Validation - `make test-examples` - `git diff --check` Generated schema artifacts and generated example-output files are intentionally not committed in this PR. Some AI assistance was used in preparing this PR (Codex/GPT-5.5). @dragon-ai-agent