T-29 — extdeps C++ ABI / target data-model (v4 TASKS.md T-29; SCHEDULED → dispatch now, operator-authorized 2026-05-18). A T-4 side-branch FEEDER — hard prerequisite of T-4's cpp slice; gates T-4→T-9; a watch item (low-dependency != low-priority — if it slips the side branch goes critical). GAP: cpp - #3267