Wavepacket σ(t) = √(ℏ²t² + 4m²σ₀⁴)/(2mσ₀); σ(0) = σ₀; lim_{t→∞} dσ/dt = ℏ/(2mσ₀)

Sympy-exact symbolic witness of Gaussian-wavepacket spreading dynamics — the canonical attosecond benchmark for free-particle evolution. Setup: ℏ, t, m, σ₀ sp-symbols positive. Time-dependent width: σ(t) = √(ℏ²·t² + 4·m²·σ₀⁴)/(2·m·σ₀);…