DecoySandwich

Finite-decoy bounds bracket the infinite-decoy limit and converge onto it

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Test	What it does	Result
test_bracket_ideal	Over 0-200 km the vacuum+weak Y1 lower bound never exceeds the ideal Y1 and the e1 upper bound never falls below the ideal e1.	✅ pass
test_gain_consistency	The returned Q1 bound is Q1 = Y1muexp(-mu), the Poisson weight of Eq. (8).	✅ pass
test_tightness	As the weak decoy goes to zero the bounds converge onto the infinite-decoy values, the deviation shrinking linearly in nu as their Sec. 3.4 predicts.	✅ pass
test_vacuum_decoy_optimal	Their Sec. 3.3: at fixed nu1 a vacuum second decoy gives the tightest bounds.	✅ pass