Add LinkedIn carousel for HAD estimator announcement

11-slide deck launching the HeterogeneousAdoptionDiD estimator. Light
gradient theme with indigo accent, matching the v3.0 / dCDH / TROP
playbook. Hero arc: "Most methods require a control group. Reality
doesn't." through to the v3.3 R-parity validation against DIDHAD v2.0.0.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: igerber