test: red executable-spec for #269 — walkforward unknown-axis rejection as bare prose

The rejection must reach stderr as the prose payload alone — no
UnknownKnob(..) Debug frame around it. Fails on today's frame; the
prose fragments already pass, so the sole pinned defect is the frame.

refs #269

crates/aura-cli/tests/cli_run.rs

@@ -4604,6 +4604,61 @@ fn aura_walkforward_emits_an_unknown_axis_rejection_once() {
     }
 }
 
+/// Property (#269): the walkforward verb's unknown-axis rejection reaches a
+/// user's stderr as prose — the same register `aura sweep`'s boundary errors
+/// adopted in #247 — never wrapped in the `UnknownKnob("…")` Rust `Debug`
+/// frame. The walkforward path already carries a fully-prosed payload INSIDE
+/// that wrapper (`axis <name>: names no param of this blueprint … — see …
+/// --list-axes`), so the fix sheds the frame rather than authoring new prose:
+/// the message that reaches stderr is exactly that payload, the axis named as
+/// typed, exit still 2. Data-free — the axis is rejected at the resolution
+/// pre-flight before any window runs, so no archive and no project are needed.
+///
+/// NB: this is the post-#269 contract. The two sibling tests above
+/// (`aura_walkforward_over_a_blueprint_rejects_an_unknown_axis`,
+/// `aura_walkforward_emits_an_unknown_axis_rejection_once`) still pin the
+/// `UnknownKnob` substring — stale pins of the defective frame that the GREEN
+/// slice repoints onto the prose fragment.
+#[test]
+fn walkforward_unknown_axis_rejection_renders_as_prose_not_a_debug_frame() {
+    let bp = format!("{}/examples/r_sma.json", env!("CARGO_MANIFEST_DIR"));
+    let out = std::process::Command::new(env!("CARGO_BIN_EXE_aura"))
+        .args(["walkforward", &bp, "--axis", "nope=1,2"])
+        .output()
+        .expect("spawn aura walkforward (unknown axis)");
+    assert_eq!(
+        out.status.code(),
+        Some(2),
+        "an unknown axis stays a usage refusal (exit 2, not a panic): status={:?}",
+        out.status
+    );
+    let stderr = String::from_utf8(out.stderr).expect("utf-8");
+    // The headline — the whole defect: no Rust `Debug` frame reaches the user.
+    assert!(
+        !stderr.contains("UnknownKnob"),
+        "must not leak the `UnknownKnob(…)` Debug frame around the prose: {stderr}"
+    );
+    // The prose payload survives intact — the axis named as typed, the
+    // names-no-param wording, and the `--list-axes` pointer (the sweep
+    // sibling's register).
+    assert!(
+        stderr.contains("nope"),
+        "the prose names the offending axis exactly as typed: {stderr}"
+    );
+    assert!(
+        stderr.contains("names no param"),
+        "the prose carries the boundary payload wording: {stderr}"
+    );
+    assert!(
+        stderr.contains("--list-axes"),
+        "the prose points at the discovery surface, like the sweep sibling: {stderr}"
+    );
+    assert!(
+        !stderr.contains("panicked"),
+        "must reject cleanly, never panic: {stderr}"
+    );
+}
+
 /// E2E (acc 2): `aura mc <open blueprint.json>` is rejected with a named error + exit 2
 /// (NOT a panic / exit 101) — MC binds no axis, so a free knob has no binder.
 #[test]