nx_toolrun_trunc_gate.nx

nx_toolrun_trunc_gate.nx -- BITE-PROOF for tr_run_capture_tr's truncation discriminator (2026-08-08). SUBJECT: tr_drain_tr / tr_run_capture_tr in runtime/nx_tool_run.nx, added for debt 1786235483 -- the three legacy drain loops exit on `total >= cap` and return success with outlen[0] == cap, which is byte-for-byte identical to a child that emitted EXACTLY cap bytes. 51+ call sites. ***WHY THE FIXTURE STRADDLES THE BOUNDARY.*** A test whose child output is smaller than the cap can never reach the branch under test -- it would pass against a stub that hardcodes FIT. So the teeth are cap=N-1 (must report CUT) and cap=N exactly (must report FIT). The second is the one that a trivial wrong implementation -- "buffer full therefore truncated" -- CANNOT pass. THE CHILD IS THIS GATE ITSELF (`emit <n>` writes exactly n bytes), so the fixture is deterministic and needs no sibling organ. If argv[0] does not resolve, the child exits 127 and the teeth FAIL loudly rather than passing vacuously. nx_toolrun_trunc_gate -> run the teeth nx_toolrun_trunc_gate emit <n> -> child fixture: emit exactly n bytes license_tier: ORIGINAL expect_exit: 0 No hw writes (Rule 26).