Support dictionary-style CF indexing

[Kkkakania]

Summary

• support position-based indexing such as obj.cf[{"T": 0}] for both DataArray and Dataset objects
• add a .cf.loc wrapper that translates CF names for label-based reads and assignments
• reuse the existing isel/sel mapper so one CF axis can expand to multiple concrete dimensions consistently

Closes #363.

Tests

• added coverage for DataArray and Dataset reads, .loc assignment, dask-backed inputs, and one-to-many axis mappings
• uv run --frozen --group test pytest cf_xarray/tests -q (296 passed, 1 skipped)
• uvx pre-commit run --all-files

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 87.03%. Comparing base (a9cebee) to head (58c4dfd).
⚠️ Report is 127 commits behind head on main.

Files with missing lines	Patch %	Lines
cf_xarray/accessor.py	87.50%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #661      +/-   ##
==========================================
+ Coverage   85.78%   87.03%   +1.24%     
==========================================
  Files          13       16       +3     
  Lines        2364     3347     +983     
  Branches      183      313     +130     
==========================================
+ Hits         2028     2913     +885     
- Misses        303      387      +84     
- Partials       33       47      +14     

Flag	Coverage Δ
mypy	45.95% <17.39%> (+7.41%)	⬆️
unittests	92.63% <95.23%> (-1.36%)	⬇️

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