`SetFromTensorProduct` may discard weights when promoting local qubits

[hutchpd]

Problem

In QuantumSystem.SetFromTensorProduct, promoted local qubits appear to have weights applied using WithWeights(...), but the returned value may be discarded.

If WithWeights returns a new qubit for local/non-system qubits, then calling it without assigning the result may silently lose the intended weights.

Why this matters

This could produce incorrect global wavefunctions when constructing a QuantumSystem from locally weighted qubits.

It is also an API trap because WithWeights appears to behave differently depending on whether a qubit is system-managed or local.

Suggested approach

Review the SetFromTensorProduct promotion path and ensure the result of WithWeights(...) is preserved.

For example:

sysQ = sysQ.WithWeights(weights, false);

or replace this with a clearer explicit in-place system method if that better matches the intended design.

Acceptance criteria

• Weighted local qubits retain their amplitudes after being promoted into a QuantumSystem.
• A regression test proves exact amplitude preservation after SetFromTensorProduct.
• The test fails against the old behaviour and passes after the fix.