rtx-fsi: time-resolved-honest load record — per-step sampling, interval median

The reported drag/lift were one instantaneous measure_force sample
every 10th coupled step, and the embedded-boundary surface force
carries zero-mean sign-flipping fresh-cell pressure transients at
step scale (measured: +-4,000-scale swings against a +-78 reference
while displacements matched the benchmark to 0.1%, and the window
MEDIANS sat near-physical). The record now samples the committed
field every step and records the interval median — the estimator the
CSV analysis supports; ten steps span 2-5% of a flap period, so
nothing physical is smeared.

Reporting only: measure_force reads the committed state. Verified on
both committed defaults against pre-change logs — every displacement,
conservation, and coupling digit identical; only the load lines
moved, and toward the benchmark: FSI3 drag 628.66 +- 545.11 ->
443.26 +- 148.50 (ref 460.2 +- 27.47, mid now within 3.7%), lift amp
2289 -> 668; FSI2 lift amp 52 -> 28.

Co-Authored-By: Claude Fable 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01X2GmJXeQ2njUecEKiJZ1G2

crates/specialized/rtx-fsi/tests/fsi2_harness/mod.rs

@@ -284,6 +284,20 @@ pub fn mid_amp(series: &[f64]) -> (f64, f64) {
     (0.5 * (max + min), 0.5 * (max - min))
 }
 
+/// Median of a sample buffer (sorts in place; empty buffers read 0).
+pub fn median(samples: &mut [f64]) -> f64 {
+    if samples.is_empty() {
+        return 0.0;
+    }
+    samples.sort_by(|a, b| a.partial_cmp(b).unwrap());
+    let n = samples.len();
+    if n % 2 == 1 {
+        samples[n / 2]
+    } else {
+        0.5 * (samples[n / 2 - 1] + samples[n / 2])
+    }
+}
+
 /// Frequency from linearly interpolated upward crossings of the mean.
 pub fn crossing_frequency(times: &[f64], series: &[f64]) -> Option<f64> {
     let (mean, _) = mid_amp(series);