Automatically reindex and align phases to a common origin

[tjlane]

! This PR was vibe-coded.

Stack

Part 9 of 11 for #31. Depends on #358. Merge bottom-up.

Next branch: codex/phase-alignment-10-alignment-options.

What this implements

Adds OriginShiftCandidate, PhaseAlignmentResult, and the first public align_phases() orchestration. It validates two isomorphous merged DataSets, automatically performs correlation reindexing when the moving metric admits an ambiguity, searches only symmetry-allowed origins, applies the selected Phenix-sign shift to a copy, and returns ranked diagnostics.

Tests

End-to-end tests cover three real space-group fixtures, automatic P 61 reindexing before origin fitting, exact phase recovery, the Phenix sign convention, result types, and nonmutation.

Reviewer focus

• The reindex-first control flow and the no-reindex path.
• The boundary between the reindexing result and residual origin search.
• The public result model and Phenix-sign behavior.

Related: #31 and #174.