N(t_{1/2})/N₀ = 1/2; with λ=ln 2, t=1 ⇒ ratio = 1/2 exactly

Sympy-exact witness of the half-life defining identity. Setup: N(t) = N₀·exp(−λt); at t = ln 2 / λ the exponent is −ln 2 exactly, giving exp(−ln 2) = 1/2. Oracle pins: half_life_symbolic = 'log(2)/lambda', ratio_at_halflife = '1/2',…