Keep the trailing auxiliary axis in a dense project

[mtfishman]

Summary

Fixes project(raw, codomain_axes, domain_axes) on a dense backend dropping a trailing dim-1 auxiliary axis (the flux-canceling leg a codomain/domain split introduces, which callers pass as reshape(a, (size(a)..., 1))). The generic allocate_project allocated a destination shaped only by the given axes, so projectto! reshaped raw down to that smaller shape and the aux leg vanished, returning a rank-2 array for a rank-3 input.

The generic allocate_project now handles a trailing surplus axis the way the graded and TensorMap backends already do: it appends that axis, taken from raw itself, to the domain, so the result's rank matches raw's. More than one trailing surplus axis throws. This lets the same project / project_aux / project_pair construction build states and charge-shifting operators on dense sites without a caller-side workaround to re-add the dropped axis.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.79%. Comparing base (d54e11f) to head (63551ff).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #217      +/-   ##
==========================================
- Coverage   81.93%   81.79%   -0.15%     
==========================================
  Files          28       28              
  Lines        1002     1005       +3     
==========================================
+ Hits          821      822       +1     
- Misses        181      183       +2     

Flag	Coverage Δ
docs	21.83% <0.00%> (-0.07%)	⬇️

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