test: add QUBO exchange parity harness

[doomhammerhell]

Summary

Adds the v0.2 QUBO exchange parity harness for the external solver boundary.

The PR verifies that, for small fixtures, all of these energy evaluators agree over every assignment:

• original QuboModel.evaluate;
• canonicalized QuboModel.evaluate;
• structured noetheris.qubo.exchange.v1 payload evaluation;
• optional local Ocean dimod.BinaryQuadraticModel.energy when Ocean is installed.

It also documents the exchange energy equation and clarifies that larger export-only models remain valid serialization artifacts without invoking the bounded exhaustive solver.

Refs #1.

Validation

python3 -m pytest tests/test_qubo_exchange_parity.py tests/test_qubo.py tests/test_optional_integrations.py
bash scripts/run_audit.sh

Local result: bash scripts/run_audit.sh passed; pytest suite reported 49 passed.

Boundary

No D-Wave credentials, sampler calls, embedding search, hardware execution, or quantum advantage claims are introduced.