test: red executable-spec for #247 — sweep-boundary bind errors render as prose

Both blueprint-sweep bind faults (wrong-kind axis value; an open param
no axis covers) must print prose in the unknown-axis sibling's register
— axis name, kind/knob specifics, a --list-axes pointer — never the raw
Debug structs KindMismatch{..}/MissingKnob(..). Exit stays 2. Fails on
the Debug leakage present today, not on scaffolding.

refs #247

crates/aura-cli/tests/cli_run.rs

@@ -2181,6 +2181,84 @@ fn aura_sweep_real_blueprint_refuses_a_raw_form_axis_name_before_data_access() {
     let _ = std::fs::remove_dir_all(&dir);
 }
 
+/// Property (#247): every sweep-verb boundary `BindError` renders as prose in
+/// the same register as its unknown-axis sibling
+/// (`aura: axis <name>: names no param of this blueprint … — see `aura sweep
+/// <bp> --list-axes``) — never a raw Rust Debug struct. A wrong-kind axis value
+/// and an uncovered open knob are the two variants that today leak
+/// `KindMismatch { knob: …, expected: …, got: … }` and
+/// `MissingKnob("…")` verbatim to stderr; both must instead name the axis, carry
+/// the specifics (expected vs supplied kind / the unbound knob), and point at
+/// `--list-axes`, with the exit code staying 2. Data-free: both faults are
+/// caught at the axis-resolution boundary before any member runs, so the test
+/// needs no archive and no project (hostless synthetic sweep).
+#[test]
+fn sweep_boundary_bind_errors_render_as_prose_not_debug_structs() {
+    let dir = temp_cwd("sweep-bind-error-prose");
+
+    // (a) KindMismatch — an axis re-opens the bound I64 `fast.length` (#246) but
+    // supplies F64 values: the specifics are the expected vs supplied kind.
+    let closed = format!("{}/examples/r_sma.json", env!("CARGO_MANIFEST_DIR"));
+    let km = Command::new(BIN)
+        .args(["sweep", &closed, "--axis", "sma_signal.fast.length=2.5,3.5"])
+        .current_dir(&dir)
+        .output()
+        .expect("spawn aura sweep (kind-mismatched axis)");
+    assert_eq!(
+        km.status.code(),
+        Some(2),
+        "a kind-mismatched axis value stays a usage refusal: status={:?}",
+        km.status
+    );
+    let km_err = String::from_utf8_lossy(&km.stderr).into_owned();
+    assert!(
+        !km_err.contains("KindMismatch"),
+        "must not leak the raw Rust Debug struct name: {km_err}"
+    );
+    assert!(
+        km_err.contains("sma_signal.fast.length"),
+        "the prose names the offending axis exactly as typed: {km_err}"
+    );
+    assert!(
+        km_err.contains("I64") && km_err.contains("F64"),
+        "the prose carries the expected vs supplied kind specifics: {km_err}"
+    );
+    assert!(
+        km_err.contains("--list-axes"),
+        "the prose points at the discovery surface, like the unknown-axis sibling: {km_err}"
+    );
+
+    // (b) MissingKnob — the open fixture leaves `slow.length` unbound when only
+    // `fast.length` is swept: the specific is the uncovered open knob.
+    let open = format!("{}/tests/fixtures/r_sma_open.json", env!("CARGO_MANIFEST_DIR"));
+    let mk = Command::new(BIN)
+        .args(["sweep", &open, "--axis", "sma_signal.fast.length=2,4"])
+        .current_dir(&dir)
+        .output()
+        .expect("spawn aura sweep (uncovered open knob)");
+    assert_eq!(
+        mk.status.code(),
+        Some(2),
+        "an uncovered open knob stays a usage refusal: status={:?}",
+        mk.status
+    );
+    let mk_err = String::from_utf8_lossy(&mk.stderr).into_owned();
+    assert!(
+        !mk_err.contains("MissingKnob"),
+        "must not leak the raw Rust Debug struct name: {mk_err}"
+    );
+    assert!(
+        mk_err.contains("sma_signal.slow.length"),
+        "the prose names the unbound knob no axis supplies: {mk_err}"
+    );
+    assert!(
+        mk_err.contains("--list-axes"),
+        "the prose points at the discovery surface, like the unknown-axis sibling: {mk_err}"
+    );
+
+    let _ = std::fs::remove_dir_all(&dir);
+}
+
 /// Property (#210 c0110 fieldtest, "refused dissolved sweeps still register
 /// their generated campaign document" — store litter, probe P3): binding only
 /// a SUBSET of the blueprint's open axes (here: `fast.length`, leaving