perf(cli): synthetic walkforward pre-flight validates axes without running members

blueprint_walkforward_family's pre-flight executed the full first-IS-
window sweep via blueprint_sweep_over and discarded the result, purely
to surface axis errors once before the parallel window fan-out (#177).
validate_axis_grid now drives the sweep terminal's own resolve/arity/
kind checks with a constant placeholder report — same single-sourced
rejection wording, no member run, no roller derivation needed.

Scope correction against the issue text: this path serves only the
synthetic (non---real) walkforward. The --real trunk routes through
verb_sugar into aura-campaign's [Sweep, WalkForward] process, whose
leading full-window sweep is a separate, larger duplication — re-filed
with corrected attribution in the issue's closing comment.

closes #253

crates/aura-cli/src/main.rs

@@ -2098,6 +2098,52 @@ fn run_oos_blueprint(
     (Vec::new(), report)
 }
 
+/// A constant, zero-compute `RunReport` for [`validate_axis_grid`]'s sweep-terminal
+/// probe. `SweepBinder::sweep_with_lattice`'s own `resolve_axes`/arity/kind checks all
+/// run BEFORE this closure is invoked per grid point, so its body never influences the
+/// validation outcome — only its signature (`Fn(&[Cell]) -> RunReport`) needs to
+/// satisfy the terminal, at O(1) cost per point instead of a full member run.
+fn axis_grid_probe_report() -> RunReport {
+    RunReport {
+        manifest: RunManifest {
+            commit: String::new(),
+            params: Vec::new(),
+            defaults: Vec::new(),
+            window: (Timestamp(0), Timestamp(0)),
+            seed: 0,
+            broker: "wf-axis-preflight-placeholder".to_string(),
+            selection: None,
+            instrument: None,
+            topology_hash: None,
+            project: None,
+        },
+        metrics: RunMetrics { total_pips: 0.0, max_drawdown: 0.0, bias_sign_flips: 0, r: None },
+    }
+}
+
+/// Validate the `--axis` grid against `doc`'s wrapped param space WITHOUT running any
+/// member (#253). Reuses the SAME strict, erroring axis-name check
+/// `blueprint_sweep_over` performs (`override_paths` — single-sourced, so the two
+/// paths cannot drift to differently-worded rejections) to derive the #246 override
+/// set, then drives the sweep terminal's own `resolve_axes`/arity/kind checks
+/// (`SweepBinder::sweep_with_lattice`) with [`axis_grid_probe_report`] standing in for
+/// the run closure — no data access, no sim engine tick. Axis resolution is
+/// window-agnostic, so the caller derives or passes no window at all.
+fn validate_axis_grid(
+    doc: &str, axes: &[(String, Vec<Scalar>)], raw_space: &[ParamSpec], probe_signal: &Composite,
+    env: &project::Env,
+) -> Result<(), BindError> {
+    let overrides = override_paths(axes, raw_space, probe_signal).map_err(BindError::UnknownKnob)?;
+    let probe = blueprint_axis_probe_reopened(doc, env, &overrides);
+    let mut iter = axes.iter();
+    let (first_name, first_vals) = iter.next().expect("a blueprint walk-forward declares >= 1 axis");
+    let mut binder = probe.axis(first_name, first_vals.clone());
+    for (n, vals) in iter {
+        binder = binder.axis(n, vals.clone());
+    }
+    binder.sweep_with_lattice(|_| axis_grid_probe_report()).map(|_| ())
+}
+
 /// The loaded-blueprint IS-refit walk-forward: per IS window, re-optimize the
 /// blueprint over the user `--axis` grid, select by `sqn_normalized`, run the
 /// winner OOS, reusing the generic `walk_forward` driver + `select_winner`;
@@ -2147,15 +2193,10 @@ fn blueprint_walkforward_family(
     // (which resolves its axes a single time before any member runs). `walk_forward` fans
     // the per-window closure out across the windows in parallel, so a `BindError` raised
     // *inside* the closure would `eprintln!`+`exit(2)` from several windows before any one
-    // exit lands — a racy, duplicated rejection (#177). Axis resolution is window-agnostic,
-    // so pre-flighting the first IS window here surfaces the error exactly once (it fails in
-    // `resolve_axes`, before any member runs); a per-window resolve then cannot re-raise.
-    let first_is = WindowRoller::new(span, is_len, oos_len, step, RollMode::Rolling)
-        .expect("roller config validated just above")
-        .next()
-        .expect("roller yields >= 1 window (validated above)")
-        .is;
-    if let Err(e) = blueprint_sweep_over(doc, axes, first_is.0, first_is.1, data, env, &binding) {
+    // exit lands — a racy, duplicated rejection (#177). Axis resolution is window-agnostic
+    // (#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:?}");
         std::process::exit(2);
     }