nx_endgame_gate.nx

nx_endgame_gate.nx -- KAT + negative controls for endgame last-piece racing. Native sovereign lane; exit 0 = all pass, N = assertion N failed. Prints measured values. T1 in-flight refcount: eg_mark/eg_clear are exact and floor at 0 T2 NORMAL pick avoids an in-flight piece (no duplicate work) -> next-rarest T3 NEG-CONTROL: NOT endgame (many pieces outstanding) -> never duplicate (-1), don't waste bw T4 ENDGAME (few obtainable left, all in-flight) -> RACE the rarest in-flight needed piece T5 NEG-CONTROL: in endgame, still only race a piece THIS peer HAS (peer-filter holds) T6 NEG-CONTROL: a piece NO peer has (avail 0) is NEVER raced (honest 99.x% boundary) -> -1