[WIP] Add flux-canceling randn(sector, inds) constructors

[mtfishman]

Summary

Adds flux-canceling constructors randn/rand/zeros(c, codomain[, domain]) at the Index level, so an ITensor over graded (block-sparse) indices can be built carrying a nonzero total charge c. They mint a fresh auxiliary Index carrying c and forward to the map-shaped split constructor, appending the aux to the domain dangling last so the physical indices fuse to c. The sector may be a bare TensorKitSectors.Sector or a SectorRange. These live in a new ITensorBaseGradedArraysExt, since ITensorBase does not depend on the sector types.

Builds on ITensor/GradedArrays.jl#220 and ITensor/TensorAlgebra.jl#216.

TODO

• Remove the [sources] pins (GradedArrays in test/, TensorAlgebra in the root) once [WIP] Add flux-canceling randn(sector, axes) constructors GradedArrays.jl#220 and Add ones_map and fill_map split constructors TensorAlgebra.jl#216 register.
• Decide whether to also cover ones/fill at the Index level, which would first need ITensorBase split ones/fill forms (deliberately absent today).

[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.19%. Comparing base (e4396ff) to head (d93ed75).

Files with missing lines	Patch %	Lines
ext/ITensorBaseGradedArraysExt.jl	60.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #230      +/-   ##
==========================================
- Coverage   77.88%   75.19%   -2.70%     
==========================================
  Files          29       29              
  Lines        1709     1701       -8     
==========================================
- Hits         1331     1279      -52     
- Misses        378      422      +44     

Flag	Coverage Δ
docs	27.09% <0.00%> (-0.17%)	⬇️

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