[WIP] Rework NamedTensorOperator around output/input pairing

[mtfishman]

Summary

Reworks NamedTensorOperator to describe an operator as a wrapped tensor plus a pairing of its dimension names into an output side and an input side, renaming the codomainnames/domainnames accessors to outputnames/inputnames. The rename frees codomain/domain for the tensor's own structural split, which is separate from the operator's output/input pairing.

The pairing is now stored as two equal-length name vectors over the wrapped tensor instead of a Bijection. Lookups are a linear findfirst, which is faster than hashing for the handful of paired legs an operator carries and drops the OrderedCollections dependency. outputname/inputname look up a wire's other end, with a three-argument get-style form that returns a supplied default when a name is unpaired (so a caller can write inputname(op, i, i) for "the other end of the wire, or i itself") and a two-argument form that throws instead.

apply(x, y) applies x to y, landing x's inputs on y's outputs (or a plain state's legs) and relabeling each consumed output of x back to its input, so the result sits on x's input space. Applying an operator to a state gives a state and applying it to another operator gives an operator, with a disjoint part of x tensored in. check_apply rejects landing an input of x on an input of y. apply_dag is removed, since the dagger application is apply(adjoint(x), y).

product(a, b) composes operators on matching sites: a name that is an input of both operands is welded through a fresh bond so the site composes, while dangling names such as batch and Kraus dimensions contract the way * contracts them. The rule that keeps a product well defined is factored into check_product: a wire of a and a wire of b must either be the same wire, which welds, or share no name, which stays independent. Connecting two operators end to end through a shared bond is left to *, which already returns the composed operator.

Priming is demoted from a pairing mechanism to a naming convenience. The pairing is explicit data, so prime/noprime survive only as name generators for constructing operators, and any pair of distinct names describes a wire equally well.

TODO

• Check for and migrate downstream users of the renamed codomainnames/domainnames accessors.

[codecov]

Codecov Report

❌ Patch coverage is 81.25000% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.67%. Comparing base (a0e9e98) to head (28ae714).

Files with missing lines	Patch %	Lines
src/namedtensoroperator.jl	81.25%	18 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #222      +/-   ##
==========================================
- Coverage   76.13%   73.67%   -2.46%     
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  Files          29       28       -1     
  Lines        1697     1683      -14     
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- Hits         1292     1240      -52     
- Misses        405      443      +38     

Flag	Coverage Δ
docs	26.26% <23.15%> (-0.49%)	⬇️

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