Check when alternative indexing or origin shifts are possible

[tjlane]

! This PR was vibe-coded.

Stack

Part 1 of 11 for #31. Merge this stack from the bottom upward.

Next branch: codex/phase-alignment-02-reindex-core.

What this implements

• Adds has_reindexing_ambiguity(), using both the unit-cell metric and space group, with an explicit obliquity tolerance and proper-hand-only scope.
• Adds has_origin_shift_ambiguity(), covering continuous polar freedom and distinct discrete origin classes after quotienting lattice and centering translations.
• Adds the shared input error, public exports, API documentation, and a small package import-order fix needed for resolvable DataSet annotations.

Tests

The tests cover exact and pseudo-merohedral metric cases, invalid metadata and tolerances, representative discrete and polar origin cases, every reference space group, all Gemmi settings, and the import-order regression.

Reviewer focus

• Whether the two predicates express the intended crystallographic definitions.
• The centering-translation quotient and strict space-group conversion behavior.
• The decision to exclude hand inversion from indexing ambiguity.

Related: #31 and #174. This prerequisite does not close the issue.

[codecov]

Codecov Report

❌ Patch coverage is 97.19626% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.23%. Comparing base (61af8b1) to head (e5acf07).

Files with missing lines	Patch %	Lines
reciprocalspaceship/algorithms/reindexing.py	90.32%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #351      +/-   ##
==========================================
+ Coverage   86.86%   87.23%   +0.36%     
==========================================
  Files          40       43       +3     
  Lines        2856     2962     +106     
==========================================
+ Hits         2481     2584     +103     
- Misses        375      378       +3     

Flag	Coverage Δ
unittests	87.23% <97.19%> (+0.36%)	⬆️

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