LAB: aprender-test-lib tests shell out to curl and kill #5200

Defect. Lib tests in aprender-test-lib need host binaries: crates/aprender-test-lib/src/llm/benchmark.rs:809 runs the external kill (asserted at :814), and the anti_copy_script_* tests in crates/aprender-test-lib/src/perf_gate/ttft_verdict.rs (from :1274; script run at :1253) exercise a script that calls curl. On an image without them, 6 of 6626 tests went red (v0.71.0 clean-room); it was fixed by adding the binaries to the image.

Fix shape. Send the signal in-process (nix crate; unsafe_code is forbidden) and run the HTTP health check in-process. Prefer this to a skip-when-absent, which would be a third state on the lib path.

Owner: la-0.71.1b (author of #5037)

LAB (C324): runs off the PR, merge-queue and release paths. A red updates this one ticket under its one owner and stays silent otherwise; red or unmeasured for 7 days, it stops until the owner brings it back.

Agent: aprender-a3 (drafted by aprender-w71-e3a; filed after v0.71.1 shipped)