Flatten lazy product operands into leaves for contraction ordering

[mtfishman]

Summary

contract_network lowered each operand to a single symbol carrying its outer axes, so a lazy product operand stayed opaque to the contraction-order optimizer. A NormNetwork's doubled vertex is such a product, lazy(ket) * lazy(conj(bra)), and collapsing it hid the physical index shared between the ket and bra. The optimizer could not interleave the other operands between the two layers, so it formed the doubled ket * conj(bra) tensor first, scaling as χ^(2·degree) rather than the χ^(degree+1) a good order reaches. A belief-propagation message update on a degree-3 vertex paid that cost on every sweep.

For a computed order, contract_network now expands each operand into its contraction leaves before ordering, so lazy(ket) * lazy(conj(bra)) reaches the optimizer as two leaves that share the physical index. The optimizer orders the full leaf set and interleaves the messages between the layers. The expansion runs on the operand after promotion to the network's common type (building on #158), so the operator and state semantics are preserved, and it recurses through nested products. An explicit order is honored over the operands as given, so it is not flattened. The Greedy optimizer and the rest of the order machinery are unchanged.

A regression test asserts the computed order's peak intermediate stays at the χ^(degree+1) path rather than forming the χ^(2·degree) doubled tensor.

[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 78.80%. Comparing base (48c5e67) to head (73c0cae).

Files with missing lines	Patch %	Lines
src/contract_network.jl	60.00%	4 Missing ⚠️

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HEAD has 12 uploads less than BASE

Flag	BASE (48c5e67)	HEAD (73c0cae)
docs	2	1
	12	1

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #159      +/-   ##
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- Coverage   84.81%   78.80%   -6.01%     
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  Files          15       13       -2     
  Lines         685      689       +4     
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- Hits          581      543      -38     
- Misses        104      146      +42     

Flag	Coverage Δ
docs	0.00% <0.00%> (ø)

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