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Phase 5c API: qkd.fock over the engine, and against its Gaussian sibling

5/5 passed in 1380ms

Test	What it does	Result
test_engine_agreement	Every number qkd.fock reports equals _core.FockState's bit for bit, on a rectangular window.	✅ pass
test_gkp_scales	The derived cutoff holds discarded under GKP_TAIL to delta = 0.15 and refuses delta = 0.08; the fixed 60 levels over-report the negativity by a third at 0.2.	✅ pass
test_grid_orientation	W[i, j] is W(xs[j], ps[i]) on a rectangular grid, integrates to 1, and matches the Gaussian layer's Wigner and Husimi pointwise with Q inside its bounds.	✅ pass
test_immutable_chain	rotate, displace and loss return new states and leave the original alone.	✅ pass
test_shadow_bridge	shadow() of Thermal(1.5) carries the (nbar + 1/2) covariance moments() hands GaussianState.from_moments, its entropy, and zero non-Gaussianity.	✅ pass