test(rtx-fsi): coupling rescue rung A' — increment trigger (CRESCUE_INC=<K>), default off

Trigger (c) of omni-cortex docs/coupling_rescue_campaign.md §11: a
predictor increment above K x its trailing-2000 median (non-rescued
steps; median refreshed every 10 steps) opens a coarse episode (rung C)
BEFORE the step's first pass — no pass is spent on a step the episode
replaces. Calibrated on the INCTRACE dumps: the anchor u=1.00 r=1 stays
under 1.72x for its whole march (K = 2..8 never fire); the (1.27, 2.0)
death crosses 3x at 145 steps before its panic and never during
resonant growth (the tip-jump trigger fires at 54). Registered K = 3,
fallback 4. Needs CRESCUE=1 and CRESCUE_COARSE=<M>.

Verified: FSI2 committed default digit-identical knob-off vs the
same-day baseline; fmt + clippy clean on the touched files.

Co-Authored-By: Claude Fable 5.1 <[email protected]>
Claude-Session: https://claude.ai/code/session_01X2GmJXeQ2njUecEKiJZ1G2

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

@@ -136,6 +136,13 @@ pub struct MarchConfig {
     /// committed tip jump. Reporting-only (rung A′'s calibration data:
     /// the healthy distribution of the increment vs a death's).
     pub inc_trace: Option<String>,
+    /// Rung A′ (`RTX_{prefix}_CRESCUE_INC=<K>`, default 0 = off; needs
+    /// `coupling_rescue` and `coarse_episode`): a predictor increment
+    /// above K × its trailing-2000 median opens a coarse episode BEFORE
+    /// the step's first pass. Calibrated (campaign doc §11): the anchor's
+    /// whole march stays under 1.72×; the (1.27, 2.0) death crosses 3×
+    /// 145 steps before its panic and never during growth.
+    pub increment_factor: f64,
     /// C^1 interface motion (constant acceleration across the step from
     /// the previous end velocity) instead of a constant velocity with a
     /// jump at the step boundary. See `Fsi2Harness::advance_subcycled`.
@@ -193,6 +200,7 @@ impl MarchConfig {
             coupling_rescue: num("CRESCUE", f64::from(u8::from(defaults.coupling_rescue))) != 0.0,
             coarse_episode: num("CRESCUE_COARSE", defaults.coarse_episode as f64) as usize,
             inc_trace: std::env::var(key("INCTRACE")).ok().or(defaults.inc_trace),
+            increment_factor: num("CRESCUE_INC", defaults.increment_factor),
             c1_interface: num("C1", f64::from(u8::from(defaults.c1_interface))) != 0.0,
             predictor: std::env::var(key("PREDICTOR")).unwrap_or(defaults.predictor),
             quiescent_release: num("QUIESCENT", f64::from(u8::from(defaults.quiescent_release)))
@@ -346,6 +354,7 @@ pub fn run_march(case: BenchmarkCase, config: &MarchConfig) -> MarchResult {
         coupling_rescue,
         coarse_episode,
         ref inc_trace,
+        increment_factor,
         c1_interface,
         ref predictor,
         quiescent_release,
@@ -488,6 +497,9 @@ pub fn run_march(case: BenchmarkCase, config: &MarchConfig) -> MarchResult {
     // step index of every episode start (for the loud cap).
     let mut coarse_remaining = 0usize;
     let mut coarse_episodes: Vec<usize> = Vec::new();
+    // Rung A′: the trailing window of predictor increments and its median.
+    let mut inc_window: std::collections::VecDeque<f64> = std::collections::VecDeque::new();
+    let mut running_inc_median: Option<f64> = None;
     let mut inc_trace_file = inc_trace.as_ref().map(|p| {
         let mut w = std::io::BufWriter::new(std::fs::File::create(p).expect("inctrace path"));
         writeln!(
@@ -765,7 +777,33 @@ pub fn run_march(case: BenchmarkCase, config: &MarchConfig) -> MarchResult {
         // accepted and counted instead of fatal.
         let retry_at = (5.0 * tol_step).max(0.1 * increment);
         let acceptable = (stall_accept * tol_step).max(0.1 * increment);
-        let mut outcome = if let Some(iqn) = iqn.as_mut() {
+        // Rung A′: the predictor increment against its trailing median —
+        // known before any pass runs, the earliest measured signature of
+        // the runaway (campaign doc §11).
+        let increment_trigger = coupling_rescue
+            && coarse_episode > 0
+            && increment_factor > 0.0
+            && step + 1 < coupled_steps
+            && running_inc_median.is_some_and(|m| increment > increment_factor * m);
+        if coupling_rescue && !increment_trigger {
+            inc_window.push_back(increment);
+            if inc_window.len() > super::rescue::TIP_JUMP_WINDOW {
+                inc_window.pop_front();
+            }
+            if inc_window.len() == super::rescue::TIP_JUMP_WINDOW && step % 10 == 0 {
+                let mut sorted: Vec<f64> = inc_window.iter().copied().collect();
+                sorted.sort_by(|a, b| a.partial_cmp(b).unwrap());
+                running_inc_median = Some(sorted[sorted.len() / 2]);
+            }
+        }
+        let mut outcome = if increment_trigger {
+            // No pass is spent on a step the episode will replace.
+            Ok(rtx_fsi::Converged {
+                state: Vec::new(),
+                residual: 0.0,
+                iterations: 0,
+            })
+        } else if let Some(iqn) = iqn.as_mut() {
             iqn.set_tolerance(tol_step).unwrap();
             iqn.solve(&d_predicted, pass)
         } else {
@@ -866,6 +904,9 @@ pub fn run_march(case: BenchmarkCase, config: &MarchConfig) -> MarchResult {
         // Rung C: (trigger, its magnitude, the rejected tip) when a coarse
         // episode is to start on this step.
         let mut do_coarse: Option<(&'static str, f64, f64)> = None;
+        if increment_trigger {
+            do_coarse = Some(("increment", increment, f64::NAN));
+        }
         if let (Some(line), Err(_)) = (&trace_line, &outcome) {
             // A stall's record prints here (accepted or fatal — a death
             // panics below, before the commit); a converged step's

crates/specialized/rtx-fsi/tests/turek_hron_fsi2.rs

@@ -164,6 +164,7 @@ fn fsi2_flapping_flag() {
             coupling_rescue: false,
             coarse_episode: 0,
             inc_trace: None,
+            increment_factor: 0.0,
             c1_interface: false,
             predictor: "structure".into(),
             quiescent_release: false,

crates/specialized/rtx-fsi/tests/turek_hron_fsi3.rs

@@ -136,6 +136,7 @@ fn fsi3_added_mass_flag() {
             coupling_rescue: false,
             coarse_episode: 0,
             inc_trace: None,
+            increment_factor: 0.0,
             // Continuous interface velocity across step boundaries: the
             // constant-velocity closure's jump at each step start drew an
             // impulsive added-mass load the unit-ratio flag could not