nx_chkarith_jo_gate.nx

nx_chkarith_jo_gate.nx -- THE PROOF GATE for LN18 (x86ctx_ovf_fuse): checked integer arithmetic at add+jo cost. Sibling of nx_bck_iv_gate, built from the same parts (nx_ccbuild_lib, gv_*): sources assembled at RUNTIME under /tmp, compiled by the compiler named in argv[1] (there is NO default compiler), the emitted ASSEMBLY as the oracle for "the long form is gone", and overflow WITNESSES whose only protection is the trap the fuse was asked to keep. WHAT THE FUSE MUST DO (measured on the /compare/lang receipt 2026-09-01: the daily mode cost 2.4x the default build because every checked add was six instructions with spills): under --chkarith the LN1 shape r = a + b; x1 = a ^ r; x2 = b ^ r; a1 = x1 & x2; c = a1 < 0; br_cond c becomes `addq` + `jo`. F1 a checked accumulate loop (sum 1..1000 = 500500, mod 251 = 6): the --chkarith assembly carries jo/jno branches, the plain assembly carries none, and the checked build adds NO xorq to the plain one (every checked add and sub site fused -- a single surviving long form would show as +2 xorq); exit 6 under plain, --chkarith AND --mode=daily (the fuse composes with the full daily bundle). F2 an add that overflows i64 at run time (MAX + argc): traps 72 under --chkarith, wraps to 91 plain. F3 a sub that overflows (MIN - argc): traps 72 under --chkarith, wraps to 159 plain. F4 __wrap_add(MAX, argc): the declared wrap-around intent stays exempt, exit 91 under --chkarith. gv_bite ties the halves: the trap fires on F2 and stays silent on F1 under the same flag. The overflow fixtures take their second operand from argc so no constant folds (the parser REFUSES a constant overflow at parse time, which would test the wrong rung). Products land in _build/ because NAS /tmp is mounted noexec (the sibling gate's measured lesson). license_tier: ORIGINAL No hw writes (Rule 26).