tests: add pendant sumtree roundtrip wip

[CharlesCNorton]

Summary

• add one WIP test derived from the high-level multi-number and top-cord summation path in khipu.v
• package dependent digit vectors, proof-carrying pendants, a heterogenous sigma ledger, and nested sumtree validation into a single extracted module
• wire the new case into tests/wip/dune

Source Shape

• reduced from the theorem surface around multi-number roundtripping and hierarchical pendant summation
• keeps decode_multi_encode_multi_roundtrip, certified pendants carrying a roundtrip witness, a heterogenous Ledger, PendantGroup, and nested SumTree validation
• includes concrete sample data for multi-number decoding, top-equals-sum checks, leaf-total checks, tree depth, and ledger population

Why This Case

• exercises dependent Vector.t payloads together with erased proof fields, sigma packaging, optional folds, and nested list/tree structure in one module
• the .t.cpp is non-empty and checks extracted wrappers directly
• the failure comes from generated C++ reached by the certified-pendant and sumtree path, not from theorem-only surface

Batch Size

• wip: +1

New WIP Tests (1)

• pendant_sumtree_roundtrip

Validation

• 1 WIP alias failed in a fresh ext4 WSL worktree after rerunning to materialize generated files
• failure occurs in generated C++ during compilation
• the emitted code breaks on the dependent optional-fold path, with generated fold_right lambdas over std::optional<...> failing the MapsTo constraints together with std::nullopt versus optional return-type mismatches in the extracted decode_multi, group_sums_validb, and sumtree_leaf_total path