Function strict_guard_mode 

pub fn strict_guard_mode() -> bool

True when OMNILUA_GC_STRICT_GUARD=1: the silent no-active-heap fallback arms (GcRef::new → detached allocation, GcRef::downgrade → forever-upgrading weak handle, GcRef::account_buffer → dropped charge) panic with a backtrace instead of degrading, so every guard-coverage gap self-reports under the existing test suites. The dual of LUA_RS_GC_QUARANTINE: quarantine turns freed-too-early into a loud failure, strict-guard turns never-freed into one (issue #249’s class).