feat(numbering): publish canonical numbering API

Add a public numbering module and route fragmenting, validation,
and scaffold preparation through the canonical numbering entry.

Rewrite the repository entry docs around the fixed numbering
contract, add MkDocs landing pages, and document the mirror
mapping used for medicinal-chemistry comparisons.

Also refresh the validation analysis reports to explain the
canonical-versus-mirrored numbering relationship.

tests/test_scripts_and_docs.py

@@ -285,6 +285,10 @@ def test_analyze_validation_fragment_library_script_generates_reports(tmp_path):
     report_zh = (output_dir / "fragment_library_analysis_report_zh.md").read_text(encoding="utf-8")
     assert "桥连或双锚点侧链不会进入当前片段库" in report_zh
     assert "cyclic single-anchor side chains" in report_zh
+    assert "6 → 13" in report_zh
+    assert "7 → 12" in report_zh
+    assert "15 → 4" in report_zh
+    assert "16 → 3" in report_zh
 
 
 def test_active_text_assets_do_not_reference_legacy_api():