nx_staghyg_gate.nx

nx_staghyg_gate.nx -- prove the staging-hygiene guard REFUSES a backwards promote, does NOT refuse a legitimate one, and -- the tooth this gate earned the hard way -- does NOT refuse an ORDINARY EDIT. Inherits the nx_gate_verdict base class (D001/L009: gv_ctr/gv_head/gv_check/gv_bite/gv_verdict) so nx_gate_green can judge it and it records a harness frame. DETERMINISTIC BY CONSTRUCTION: every fixture mtime is set explicitly via sys_utimensat and `now` is passed to sg_classify as a parameter, so there are NO sleeps and NO filesystem-resolution races. Each tooth writes its OWN file names, so nothing depends on whether the write helper truncates. THE cfg BELOW IS THE SHIPPED POLICY, NOT A CONVENIENT ONE. An earlier version of this gate passed 10/10 against thresholds the shipped conf did not use, which proves the mechanism and says nothing about the rule operators actually meet. T4 and T8 prove it FIRES; T11 proves it STAYS SILENT on a one-run edit -- and T11 exists because the first tuning refused the very build that introduced it. A guard that fires on routine work is not strict, it is training people to bypass it. license_tier: ORIGINAL No hw writes (Rule 26). expect_exit: 0