Family: s-unit — S-unit equation

[roed-math]

One family: S-unit equation (s-unit) — P2, algorithmic.

• Standard form: a*u + b*v = c, u, v S-units
• Parent: exponential-diophantine

Contents: the registry entry, its docs/FAMILIES.md section and its rows in the classification corpus.

References cited (the claim each one supports):

• deWeger1987 — the practical LLL-based algorithm
• Alvarado2021 — the Sage implementation for arbitrary number fields

Independent of every other family PR: it needs only the registry backbone it is based on.

Part of the series that splits #1 into reviewable pieces:

1. 01-bibliography — packaging, docs, annotated bibliography
2. 02-parsing — equation strings to a term model
3. 03-registry — the YAML family registry (3 seed families)
4. 04-matchers — shape recognizers
5. 05-classify — the classification pipeline
6. 06-solvers — solver framework, two seed solvers, and the CLI
7. 07..09-backbone — the 23 parent families of the DAG, by depth
8. one PR per remaining family (38 of them, mutually independent)
9. 99-polish — restore the full doctests and tighten the invariants