Balanced gram_eigh_full convention, add one and operator-construction primitives

[mtfishman]

Summary

Flips gram_eigh_full from the right-Gram (A ≈ X' * X) convention to the balanced left form (A ≈ X * X'), where X = V * sqrt(D) is the symmetric eigendecomposition with the square-root absorbed into the eigenvector matrix. The returned factor X now carries the codomain axes followed by a trailing rank axis, matching the layout of the eigendecomposition's V and the SVD's U, and parallel to the L-form of Cholesky. The companion gram_eigh_full_with_pinv now returns a left inverse Y such that Y * X ≈ I on the rank subspace.

Also adds TensorAlgebra.one(A, codomain, domain) for constructing an identity operator tensor with the same shape as A, with the layered dispatch chain (labels, perm tuples, biperms, Val{ndims_codomain}) used by the other factorizations. Not exported because exporting would clash with Base.one. Qualify as TensorAlgebra.one(...).

Adds TensorAlgebra.similar_map(prototype, [T,] codomain_axes, domain_axes) for allocating an array shaped as a linear map with axes (codomain_axes..., domain_axes...). This is the primitive NamedDimsArrays.similar_operator will route through (ITensor/NamedDimsArrays.jl#229), so a backend only needs to overload similar_map to plug into the named-axis wrapper.

Adds projectto!(dest, src) and checked_projectto!(dest, src; atol, rtol) for orthogonal projection of a source array into the representable subspace of dest. projectto! drops the non-representable component without looking at its size. checked_projectto! runs the projection and verifies the discarded weight via isapprox, throwing on tolerance violations. The default projectto! falls through to copyto!, correct for dense backends where everything is representable. AbelianGradedArray (in GradedArrays.jl) overloads it to copy only symmetry-allowed entries.

Pairs similar_map with projectto! as project_map(raw, cod, dom) = projectto!(similar_map(raw, cod, dom), raw), the data-bearing sibling of the _map allocator family. checked_project_map is the same on top of checked_projectto!. Operator constructors call the checked variant by default.

The gram_eigh_full convention change is technically breaking, but the function was introduced very recently and has no known downstream users, so this lands as a patch bump (0.9.5).

[codecov]

Codecov Report

❌ Patch coverage is 65.62500% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.03%. Comparing base (0895aa9) to head (8409c0d).

Files with missing lines	Patch %	Lines
src/projectto.jl	0.00%	9 Missing ⚠️
src/factorizations.jl	88.23%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #177      +/-   ##
==========================================
- Coverage   80.61%   80.03%   -0.58%     
==========================================
  Files          24       26       +2     
  Lines        1011     1037      +26     
==========================================
+ Hits          815      830      +15     
- Misses        196      207      +11     

Flag	Coverage Δ
docs	26.47% <53.12%> (+0.74%)	⬆️

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