Add pretty printers for Statevec and DensityMatrix

[donglrd]

Summary\n- add complex-number pretty formatting for fractions, square-root constants, and phase exponentials\n- add statevector ket expansion rendering via Statevec.draw()\n- add density-matrix outer-product rendering via DensityMatrix.draw()\n- cover the new formatting helpers and draw methods with tests\n\nCloses #501\n\n## Testing\n- pytest tests/test_pretty_print.py -q\n- python3 -m ruff check graphix/pretty_print.py graphix/sim/statevec.py graphix/sim/density_matrix.py tests/test_pretty_print.py

[thierry-martinez]

Thank you for your contribution! As noted in the preamble of #501, this issue is part of unitaryHACK 2026; therefore, contributions will be evaluated during the hackathon period, from June 3 to June 17, 2026.