[RaphaelFakhri]

MappingIter stops once its offset reaches Mapping::len, the number of stored values. When the ids in a Mapping are not contiguous from zero, or a value was removed with unset, the iterator ends before it reaches the highest id and silently skips entries.

DependencySnapshot::from_provider hits this when the snapshot covers only some of the provider's packages. It iterates packages to apply the provider's candidate order, so packages with high NameIds keep order: 0 for every solvable. Solving against the snapshot then picks candidates in insertion order instead of the provider's preferred order, which can produce a different solution than the original provider.

Changes

• MappingIter now iterates up to the highest inserted id (Mapping::max) instead of Mapping::len.
• test_iter_sparse in src/utils/mapping.rs covers sparse ids and unset.
• test_snapshot_subset_of_packages_keeps_candidate_order snapshots only b from a provider that also has a. Before the fix the snapshot solves to b=1; with the fix it solves to b=3, matching the original provider's highest-version-first order.

Testing

cargo test --lib mapping
cargo test --test solver test_snapshot_subset
cargo test --workspace --all-features
cargo fmt --all -- --check
cargo clippy --workspace --all-targets --all-features -- -D warnings

Fixes #318