test(rtx-fsi): coupling-level rescue rung A (RTX_FSI{2,3}_CRESCUE, default off) + TRACE_FROM autopsy window

Coupling-rescue campaign (omni-cortex docs/coupling_rescue_campaign.md).
Every recorded FSI2 s=1 coupling death is the END of a multi-step
runaway (74-march burst scan: green marches never exceed 2.4x their own
p95 tip jump; 27/28 deaths burst 5-37 steps first). The traced autopsy
of the (1.27, 2.0) death (replay digit-identical, CSV byte-identical)
shows a growing period-2 instability of the CONVERGED coupled scheme at
the tip's max-velocity crossing: increment x33 and converged load
1e4 -> 1e6 N over 200 steps with the coupling converging on 111 of the
first 112 steps.

- march.rs: RTX_{prefix}_TRACE_FROM (print-only autopsy window: per-pass
  residual/load + one line per step with increment, tol_step, retry_at,
  acceptable, outcome, committed tip jump); RTX_{prefix}_CRESCUE (default
  off): on a fatal stall or a committed tip jump > 3x the running p95
  (trailing 2000 non-rescued steps), reject the step and repeat the
  interval as 2/4/8/16/32 coupled substeps of dt/n (Mayr-Wall-Gee
  reduced-step repetition, five repetitions); per-rescue record printed,
  MarchResult.coupling_rescues/_failures/rescue_records, rate cap 20
  rescues per second of march (dies loudly).
- rescue.rs (new): the substep ladder — each substep a complete coupled
  step at dt/n with its own predictor, fresh coupler, C1 velocity
  chaining, tolerances at the substep's increment; the march's own step
  path is NOT routed through it (digit identity by construction).
- rtx-fea NonlinearDynamicStepper::step_with_dt (step at an explicit dt;
  step() delegates float-for-float); Fsi2Harness::advance_subcycled_with
  (explicit fluid dt; advance_subcycled delegates).

Verified: FSI2 + FSI3 committed defaults and the noise probe
digit-identical knob-off vs same-day / 2026-08-31 baselines; knob ON
on the anchor u=1.00 r=1: green, zero rescues, CSV byte-identical to
TWIN-1's u1.00.csv; fmt + clippy clean on the touched files.

Verdict of rung A on the provocation set: REFUTED 3/3 by mechanism —
every rescued interval was carried but reproduced the rejected step's
motion (dt/2..dt/32 give the same jump), so the runaway lives in the
coupled LOAD at the crossing, not in the time integration; the
SUBCYCLE=2 closure marches through the same crossing. The knob stays,
default off, as the instrument that measured this.

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

crates/specialized/rtx-fea/src/analysis/nonlinear_dynamic.rs

@@ -495,7 +495,24 @@ impl<'a> NonlinearDynamicStepper<'a> {
     /// Rescued steps are counted in [`Self::rescue_counts`]; if every
     /// level fails, the plain loop's original error is returned.
     pub fn step(&mut self, state: &DynamicState) -> FeaResult<(DynamicState, usize)> {
-        let dt = self.analysis.dt;
+        self.step_with_dt(state, self.analysis.dt)
+    }
+
+    /// [`Self::step`] over an explicit interval `dt` instead of the
+    /// analysis's own — the same plain Newton, line search and
+    /// subdivision ladder (every level already takes `dt` as a
+    /// parameter; the Newmark mass term follows it). The analysis's
+    /// `dt` path is exactly `step`, float for float. A coupled march's
+    /// coupling-level rescue uses this to repeat an interval as `n`
+    /// substeps of `dt/n`.
+    ///
+    /// # Errors
+    /// As [`Self::step`].
+    pub fn step_with_dt(
+        &mut self,
+        state: &DynamicState,
+        dt: f64,
+    ) -> FeaResult<(DynamicState, usize)> {
         match self.newmark_newton(state, dt, false) {
             Ok(result) => Ok(result),
             Err(first_failure) => {