Preserve the codomain/domain split in map construction and aligndims

[mtfishman]

Summary

Map-shaped construction now forwards a codomain/domain split down to the backing storage instead of flattening. randn, rand, and zeros take a codomain and a domain index tuple (randn((i,), (j,))), and the rand_map/randn_map/zeros_map hooks on indices build a backing object partitioned accordingly: a TensorKit TensorMap stores a codomain ← domain map, a dense backend stores flat. Following the similar_map convention the domain is conjugated in the flattened view, so the result is an ordinary tensor over the codomain indices and the duals of the domain indices. This fixes rand_map((i,), (j,)) on TensorMap-backed indices, which previously flattened everything into the codomain.

aligndims gains a map form aligndims(a, codomain, domain) that both repartitions and permutes, and its existing single-tuple form now routes through TensorAlgebra.permutedims so a TensorMap-backed tensor permutes correctly (a TensorMap has no flat permutedims). Both ride the permutedims seam from ITensor/TensorAlgebra.jl#202, so any backend implementing it works with no dedicated method, and a dense backend ignores the split and stores flat.

[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 74.80%. Comparing base (091773d) to head (eb1556c).

Files with missing lines	Patch %	Lines
src/abstractnamedtensor.jl	85.71%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #207      +/-   ##
==========================================
+ Coverage   74.62%   74.80%   +0.18%     
==========================================
  Files          30       30              
  Lines        1537     1564      +27     
==========================================
+ Hits         1147     1170      +23     
- Misses        390      394       +4     

Flag	Coverage Δ
docs	23.35% <3.57%> (-0.43%)	⬇️

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