feat: flag ESO ceiling in legend and result struct

Add at_ceiling: bool to EsoResult, set when omega0_opt reaches the
stability-constrained search bound (omega0_max_stable - GSS_TOLERANCE).
Propagate through AxisEsoData into the plot legend as ' [at ceiling]'
so users can see at a glance when the optimizer hit its upper bound
rather than finding a true interior optimum.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: nerdCopter

src/eso.rs

@@ -66,6 +66,8 @@ pub struct EsoResult {
     pub b0_auto: bool,
     /// N-step-ahead prediction MSE at the optimal omega0.
     pub mse: f64,
+    /// True when omega0_opt is at the search ceiling (result may not be the true optimum).
+    pub at_ceiling: bool,
     /// Number of samples used in the optimization.
     #[allow(dead_code)]
     pub sample_count: usize,
@@ -362,6 +364,7 @@ pub fn run_eso_optimization(
         .best_param
         .ok_or("ESO optimization returned no solution")?;
     let mse = run_result.state().best_cost;
+    let at_ceiling = omega0_opt >= omega0_max_stable - ESO_GSS_TOLERANCE;
 
     let (beta1, beta2) = leso2_gains(omega0_opt);
 
@@ -376,6 +379,7 @@ pub fn run_eso_optimization(
         b0,
         b0_auto,
         mse,
+        at_ceiling,
         sample_count: omega_meas.len(),
         timestamps,
         omega_meas_trace: omega_meas,

src/plot_functions/plot_eso.rs

@@ -80,10 +80,13 @@ pub fn plot_eso_output(
         let mut series = vec![
             PlotSeries {
                 data: hat_data,
-                label: format!(
-                    "ESO estimate (omega_hat, omega0={:.0} rad/s, b0={:.2})",
-                    data.omega0_opt, data.b0
-                ),
+                label: {
+                    let ceiling = if data.at_ceiling { " [at ceiling]" } else { "" };
+                    format!(
+                        "ESO estimate (omega_hat, omega0={:.0} rad/s, b0={:.2}){ceiling}",
+                        data.omega0_opt, data.b0
+                    )
+                },
                 color: color_hat,
                 stroke_width: stroke + 1,
             },
@@ -123,6 +126,7 @@ pub fn plot_eso_output(
 struct AxisEsoData {
     omega0_opt: f64,
     b0: f64,
+    at_ceiling: bool,
     /// (time, omega_meas, omega_hat) triples
     meas_hat: Vec<(f64, f64, f64)>,
     /// (time, f_hat_raw) before visual scaling
@@ -158,6 +162,7 @@ fn build_axis_data(eso: &EsoResult) -> AxisEsoData {
     AxisEsoData {
         omega0_opt: eso.omega0_opt,
         b0: eso.b0,
+        at_ceiling: eso.at_ceiling,
         meas_hat,
         fhat,
         fhat_max_abs,