Family: genus-two — Genus 2 curve

[roed-math]

One family: Genus 2 curve (genus-two) — P1, effective.

• Standard form: y^2 = f(x), deg f in {5, 6}
• Parent: hyperelliptic
• Examples: y^2 = x^5 - x + 1

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

References cited (the claim each one supports):

• Igusa1960 — the invariants identifying a genus-2 curve up to isomorphism (the classifier's genus-2 handle)
• CasselsFlynn1996 — explicit arithmetic of genus-2 Jacobians

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