P66: Nature of the Critical Transition at Mcrit in CCEGA — First-Order Echo Amplitude and Bimodal Timescale Distribution

We characterize the phase transition at Mcrit = 14.75 M☉ from the perspective of GW echo observability. The transition is first-order in echo amplitude: h_echo = h_max for M < Mcrit (black star, perfect reflector) and h_echo = 0 for M > Mcrit (CCEGA BH, horizon). The echo timescale is continuous through Mcrit, reaching t_echo^crit ≈ 0.100 ms, then jumping to t_echo^BH ≈ 0.195 ms just above Mcrit as the echo source switches from the exterior surface to the interior surface inside the horizon. A population of mergers spanning Mcrit would show a bimodal distribution in echo timescales with a gap between ~0.10 ms and ~0.19 ms — a unique CCEGA prediction testable with Einstein Telescope.