Add benchmarks for MDAnalysis.analysis.align

[noir4201]

Fixes #1023

Changes made in this Pull Request:

• Adds new file benchmarks/benchmarks/analysis/bench_align.py
• time_alignto_all_atoms: single frame alignment on all atoms
• time_alignto_backbone: alignment on backbone selection only
• time_alignto_mass_weighted: mass weighted alignment
• time_aligntraj_all_frames: full trajectory alignment across all frames
• time_average_structure: average structure calculation

No benchmarks previously existed for the MDAnalysis.analysis.align
module. This is part of increasing benchmark coverage as outlined
in GSoC 2026 Project 3: Benchmarking and Performance Optimization.

LLM / AI generated code disclosure

LLMs or other AI-powered tools (beyond simple IDE use cases)
were used in this contribution: yes

AI was used to help structure the benchmark class and suggest
which functions to benchmark. All code was reviewed, tested
and verified locally by the contributor.

PR Checklist

• Issue raised/referenced?
• Tests updated/added?
• Documentation updated/added?
• package/CHANGELOG file updated?
• Is your name in package/AUTHORS?
• LLM/AI disclosure was updated.

Developers Certificate of Origin

I certify that I can submit this code contribution as described in the
Developer Certificate of Origin, under the MDAnalysis LICENSE.

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📚 Documentation preview 📚: https://mdanalysis--5298.org.readthedocs.build/en/5298/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.82%. Comparing base (6e837a4) to head (2803d6f).
⚠️ Report is 1 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #5298      +/-   ##
===========================================
- Coverage    93.83%   93.82%   -0.01%     
===========================================
  Files          182      182              
  Lines        22501    22483      -18     
  Branches      3195     3195              
===========================================
- Hits         21114    21095      -19     
- Misses         924      925       +1     
  Partials       463      463              

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