fix(cli): render sweep-boundary bind errors as prose, not Debug structs

The blueprint-sweep terminal rendered BindError via {:?}, leaking
KindMismatch { .. } and MissingKnob(..) Rust Debug structs where the
sibling diagnostic on the same verb ships prose. Both now render in
that register — the axis name, the expected-vs-supplied kinds resp.
the unbound knob, and the --list-axes pointer; exit stays 2.

Scoped out (same defect family, different verb path): the walkforward
path's UnknownKnob wrapper still prints its Debug-ish frame around an
already-prose payload and is pinned verbatim by two existing tests;
filed as a follow-up rather than widened into this slice.

closes #247

crates/aura-cli/src/main.rs

@@ -1915,6 +1915,31 @@ fn override_paths(
     Ok(overrides)
 }
 
+/// Renders a [`BindError`] as one-line prose in `override_paths`' sibling
+/// register above (#247) — never the raw Rust `Debug` struct name
+/// (`KindMismatch { .. }` / `MissingKnob("..")`), the two variants the sweep
+/// terminal actually raises past `override_paths`' own pre-flight (which
+/// already rejects an unresolvable axis name as prose before either call
+/// site below ever reaches the terminal). Other variants keep the
+/// established `{:?}` rendering unchanged — in particular `UnknownKnob`
+/// already carries a fully-prosed message string at both call sites here
+/// (wrapped from `override_paths`' own `Result<_, String>`), and pinned
+/// tests (`aura_walkforward_emits_an_unknown_axis_rejection_once` et al.)
+/// depend on that Debug-wrapped shape.
+fn render_bind_error(e: &BindError) -> String {
+    match e {
+        BindError::MissingKnob(name) => format!(
+            "axis {name}: an open param with no axis and no bound default — \
+             bind it with `--axis {name}=<value>` — see `aura sweep <bp> --list-axes`"
+        ),
+        BindError::KindMismatch { knob, expected, got } => format!(
+            "axis {knob}: expected {expected:?}, supplied {got:?} — \
+             see `aura sweep <bp> --list-axes`"
+        ),
+        other => format!("{other:?}"),
+    }
+}
+
 /// Apply a validated override set to a freshly loaded strategy (#246).
 /// Infallible by contract: the set was derived against this same document at
 /// the family boundary.
@@ -2015,9 +2040,9 @@ fn blueprint_sweep_family(
             // the shared reduce-mode member path — the same fn reproduction re-runs.
             run_blueprint_member(reload(doc), point, &space, data.run_sources(env, &binding.columns()), window, 0, pip, &topo, env, StopRule::Vol { length: R_SMA_STOP_LENGTH, k: R_SMA_STOP_K }, &binding, &[])
         })
-        // render the sweep terminal's BindError to a message (the fn's String error contract),
-        // so `UnknownKnob("nope")` still surfaces verbatim at the IO wrapper.
-        .map_err(|e| format!("{e:?}"))
+        // render the sweep terminal's BindError to prose (#247), the fn's String error
+        // contract — never the raw Debug struct.
+        .map_err(|e| render_bind_error(&e))
 }
 
 /// Sweep the LOADED blueprint over the user `--axis` grid on an in-sample window
@@ -2188,7 +2213,7 @@ fn blueprint_walkforward_family(
     // (#253): `validate_axis_grid` resolves the SAME grid the sweep terminal would, without
     // running a single member — no IS window (or a second roller) is needed here at all.
     if let Err(e) = validate_axis_grid(doc, axes, &raw_space, &probe_signal, env) {
-        eprintln!("aura: {e:?}");
+        eprintln!("aura: {}", render_bind_error(&e));
         std::process::exit(2);
     }
     walk_forward(roller, space.clone(), |w: WindowBounds| {