feat(toolkit): ship macro_lactone_toolkit package

Unify macrolactone detection, numbering, fragmentation, and
splicing under the installable macro_lactone_toolkit package.

- replace legacy src.* modules with the new package layout
- add analyze/number/fragment CLI entrypoints and pixi tasks
- migrate tests, README, and scripts to the new package API

tests/test_imports.py

@@ -0,0 +1,5 @@
+import macro_lactone_toolkit
+
+
+def test_public_imports_smoke():
+    assert macro_lactone_toolkit is not None