Family: mordell — Mordell equation

[roed-math]

One family: Mordell equation (mordell) — P1, algorithmic.

• Standard form: y^2 = x^3 + k
• Parent: elliptic-weierstrass
• Examples: y^2 = x^3 - 2

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

References cited (the claim each one supports):

• GebelPethoZimmer1998 — the method, with tables for |k| <= 10^4
• BennettGhadermarzi2015 — complete tables for |k| <= 10^7

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