Preserve candidate-selection metadata during resolution

[charliermarsh]

Summary

Add VersionSet::selection_eq to distinguish extensional version-set equality from candidate-selection interchangeability. Eq and Hash remain based solely on contained versions, and all solver term relations remain membership-only.

Use selection_eq only when compacting dependency incompatibilities. Dependencies with equal version membership but different candidate-selection behavior remain separate, while selection-equivalent dependencies can still be merged.

Strengthen the VersionSet contract by requiring selection_eq to be an equivalence relation and a congruence across version-set operations. Selection-equivalent ranges must also be interchangeable for dependency prioritization and version choice. Candidate-selection metadata may change which contained version is chosen, but it must not change whether a candidate exists. Existing VersionSet implementations retain their current behavior through the default self == other implementation.

PubGrub does not propagate metadata from constraints that are already satisfied by version membership. Regression coverage confirms membership-only relations for equal and strict-subset terms, preserves the decided-package invariant, distinguishes metadata-sensitive dependencies during compaction, merges structurally distinct but selection-equivalent dependencies, and retains collision-safe merging behavior.

Supports astral-sh/uv#20098.

[codspeed-hq]

Merging this PR will not alter performance

✅ 7 untouched benchmarks

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_{Comparing charlie/version-set-selection-metadata (45562e8) with main (1856af6)}