GaussianLoss

Thermal-loss channel: fixed points, sqrt(T) scaling, explicit xi plane

7/7 passed in 2ms

Test	What it does	Result
test_coherent_scaling	Pure loss scales a coherent amplitude by sqrt(T): T = 0.36 sends (1.2, -0.4) to (0.72, -0.24).	✅ pass
test_excess_noise_physical	Thermal loss at positive xi yields a bona fide state and physical() agrees.	✅ pass
test_identity_channel	thermal_loss with T = 1 and xi = 0 is the identity channel.	✅ pass
test_loss_toward_vacuum	Pure loss pulls both variances toward the vacuum 1/2, from either side.	✅ pass
test_ref_validation	An unknown ref raises ValueError and an omitted one TypeError; the plane is never defaulted.	✅ pass
test_reference_plane	xi = 0.02 at ref="input" gives the same covariance as xi = 0.008 at ref="output" for T = 0.4.	✅ pass
test_vacuum_fixed_point	The vacuum is a fixed point of pure loss at any transmittance.	✅ pass