76512f6048
barrier_never_both_no_fallback_and_stale_reader drives the REAL post-barrier handle_with_ip against a live in-process Core + a mock Python upstream that counts every request and answers with a coins=111 marker: - STALE READER: credits + userMassInfo show the Core balance, never 111 (userMassInfo proxies the Python envelope but the Rust economy overlay wins). - NEVER BOTH (Core up): every pure economy route returns a Rust body (no __python__ marker) and the Python proxy call-count stays 0. - NO FALLBACK: a server pointed at a dead Core port (built without probing Core: empty catalog + empty pool) fails closed (credits/match -> 503) and STILL never proxies to Python (call-count unchanged). Also parametrizes build_econ_server's pass URL so the mock upstream can be injected. host 71 lib + 4 integration + differential + concurrency + failure + 24 host_test all green; clippy -D warnings + fmt clean.