nx_arena_scope_test.nx

nx_arena_scope_test.nx -- smoke for mark/reset-to-mark + ceiling. Verifies the SSS-class behaviour that closes the arena issue from NISHI_HONEST_TRADE_OFFS.md: nested workloads can scope their allocations without accumulating across loop iterations. Construct the NxArena manually from a caller-provided buffer (per user directive "user owns every bit") rather than calling nx_arena_new which mmaps internally. This keeps the smoke cross-target portable -- the test doesn't depend on a particular syscall set. expect_exit: 0 license_tier: ORIGINAL