✨ Add QCO density-matrix DD simulation

[simon1hofmann]

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Summary

Adds density-matrix execution as the final layer of the QCO DD functionality stack.

• Construct density-matrix DDs from pure states.
• Simulate and sample QCO programs from density matrices.
• Evolve mixed states under supported unitary and dynamic operations.
• Deallocate entangled qubits through partial trace.
• Expose density simulation and sampling in Python.
• Consolidate interpreter plumbing and retain the complete validation and error contracts from the lower layers.

Closes #1915.

Validation

• Every layer in the stack builds and passes independently.
• All 151 QCO utility tests pass on this layer.
• All 9 focused Python QCO DD tests pass.
• Full stub generation and the repository lint suite pass.
• The final source tree is equivalent to the previously reviewed monolithic implementation, excluding intentionally removed temporary agent plans.

GPT-5.6 via Codex materially assisted with implementation, testing, review remediation, and restructuring this work into a stacked pull request under maintainer direction.