Consume the tag/decoration API and further shrink the itensors.jl shim

[mtfishman]

Summary

Continues shrinking the itensors.jl shim onto the upstream API, now that ITensorBase exposes the tag and factorization-decoration verbs (added in ITensor/ITensorBase.jl#214). Builds on #2.

• Name the new bond at the factorization instead of relabeling it after. The left_orth / right_orth call sites pass name = (; tags = ...), which mints the bond with the tag directly, so the commonind / settags / replaceinds retag that followed each factorization is gone. The full-update bond takes its tags from commonind(p, q), which errors clearly when there is not exactly one shared index, rather than trycommonind.
• Retire the local settags shim for the now-public ITensorBase.settags, and mint tagged site and link indices through the Index(dim; tags = ...) keyword constructor.
• Take the loop_correlation eigenvalues with eig_vals directly on the ITensor, dropping the matricize / adapt / Array densification and the LinearAlgebra.eigvals call. It returns the same eigenvalues, and the routine only reads maximum(abs, λ) and sum(abs, λ).
• Use the greedy contraction optimizer unconditionally for loop weights. weight picked the optimizer with hasqns (exact for graded tensors, greedy otherwise), but loop_correlation in the same module already runs greedy on every backend, so drop the branch and the hasqns helper it was the only caller of.
• Update the API reference for the removed safe_eigen and the op methods that moved into the Ops submodule.

Verified with the local symmetric smoke across the dense, graded, and TensorKit backends, the package test suite, and a docs build.