nx_assert.nx

nx_assert.nx -- runtime self-check primitives. Phase 1 of the detection stack (feedback-three-disciplines.md and the bootstrap_proof debug session of 2026-04-24 shipped the first ad-hoc guards; this file factors them out so any runtime code can assert cheaply). Pattern mirrors ASAN's FAIL_FAST: if (!INVARIANT) { sys_write(2, "tag: msg\n", N); exit(200); } Exit code 200 is "assertion failed". Distinct from 0 (success), 1-127 (typed failures), 124/137/139 (qemu timeout/SIGKILL/SIGSEGV) so debug harnesses can grep for it. Tradeoffs: * Every assert is a branch + call. Negligible perf on the success path -- branch predictor pins the not-taken leg and there are zero data-dependent loads. * Error messages emitted directly via sys_write so the assert fires even when a heap allocator is the thing that broke. Later: a release-mode compile-time flag elides the checks. The release build is expected to run the check code only in debug CI / bootstrap_proof diagnostic passes.