Add qutrit state-vector and density-matrix reference simulators

[qciaran]

Summary

• add shared uniform-local-dimension QuditStateVec and QuditDensityMatrix reference backends with qutrit aliases for |0>, |1>, |L>
• support arbitrary multi-site unitaries, embedded qubit gates, preparation, reset, full and strict computational measurements, and exact or trajectory Kraus channels
• expose Kraus branch probabilities, joint basis outcomes, coarse-grained projective partitions, and generalized measurement instruments with exact mixed conditional states
• provide pure- and mixed-state reduced density matrices, density purity, and trace, Hermiticity, and positive-semidefiniteness diagnostics
• validate imported density operators and unitary/channel completeness; use fallible dense allocation and expose memory estimates
• include device-neutral qutrit leakage and seepage channel constructors
• document radix ordering, API semantics, and O(d^N) / O(d^(2N)) scaling

Verification

• 19 focused qudit tests, including randomized d=2 through d=4 circuits, exact-versus-trajectory convergence, multi-site channels, generalized measurements, and backend cross-validation
• complete pecos-simulators test suite
• strict Clippy with warnings denied
• cargo formatting
• all pre-commit hooks

A rustdoc run with all existing warnings promoted to errors reaches unrelated pre-existing broken intra-doc links outside the new qudit module; the normal crate doctest suite passes.

This is the native simulator foundation from issue #585. GeneralNoiseModel and Selene boundary verification can follow after PR #545 lands without coupling the core simulator API to the plugin branch.

Progresses #585.