nx_apistack_bulkhead.nx

nx_apistack_bulkhead.nx -- CAP-API-BULKHEAD: concurrency isolation for /api. A bulkhead caps concurrent in-flight calls to a resource so a slow/expensive route can't exhaust the whole plane (isolation, the Titanic-compartment pattern). Pure counter: bh[0]=active. acquire returns 1 (and increments) if under max, else 0 (reject/shed). Deterministic, gateable offline. license_tier: ORIGINAL