Reorganize tree-DMRG sources top-down

Move `dmrg` to the top of `dmrg.jl` so the file reads entry-point first, matching `apply_operators.jl`. Split the generic pieces into their own files: `tensornetworkoperator.jl`, `quadraticformnetwork.jl`, `orthogonalize.jl`, and `nameddimsarrays_extensions.jl` (the temporary `VectorInterface` methods on `AbstractNamedDimsArray`).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: mtfishman

src/ITensorNetworksNext.jl

@@ -21,6 +21,10 @@ include("beliefpropagation/normnetwork.jl")
 
 include("apply/apply_operators.jl")
 
+include("nameddimsarrays_extensions.jl")
+include("tensornetworkoperator.jl")
+include("quadraticformnetwork.jl")
+include("orthogonalize.jl")
 include("dmrg.jl")
 
 end