v0.3.0: multimode comb molecule

Adds the multimode comb molecule: every retained comb line of the LLE fluctuation matrix coupled to a matching auxiliary-ring mode (molecule_fluctuation_matrix), with multi-line bus detection and joint twin-beam quadratures through the auxiliary ring (output_variance_ports now accepts several port modes and joint read-out).

Asserted in the test suite, not claimed: at one retained line the builder equals the released v0.2 two-ring matrix to machine precision; at zero coupling it reduces exactly to the plain fluctuation matrix and the v0.1 spectra (single-line and twin-beam); a passive multimode molecule returns exact vacuum through any bus; the resonant auxiliary ring at zero frequency is exactly the single ring with J^2/gamma_b extra loss per line, including the quarter-turn quadrature rotation; twin-beam squeezing through the auxiliary bus is strictly deeper than through the main bus of the same device once J^2/gamma_b exceeds one; and the stability guard refuses a flat state that is above a pair's modulational-instability threshold when the molecule's added loss is removed. 21 tests total.

Install: pip install sqzcomb