FidelityDistance

Trace distance where it is exact, and the bracket where it is not

3/3 passed in 2ms

Test	What it does	Result
test_bounds_bracket	The Fuchs-van de Graaf bracket stays ordered in [0, 1], tight on a pure pair.	✅ pass
test_mixed_refused	A mixed pair has no closed form: trace_distance raises and names trace_bounds.	✅ pass
test_pure_distance	A pure pair has $D=\sqrt{1-F}$ exactly: $\sqrt{1-\exp (-|d{|}^2/2)}$ for coherent states, 0 for identical ones.	✅ pass