test(rtx-fsi): coupling rescue rung C (CRESCUE_COARSE, burst-local s=2 coarsening) + INCTRACE per-step increment dump — both default off

Coupling-rescue campaign, continued (omni-cortex
docs/coupling_rescue_campaign.md §11). Rung A was refuted 4/4 by
mechanism (the substeps reproduce the rejected motion — the runaway is
in the converged coupled load at the crossing). Diagnostics on the same
death: SUBCYCLE=2 marches GREEN to t=16 (zero bursts), HYST=0.25 dies
EARLIER. So:

- RTX_{prefix}_CRESCUE_COARSE=<M> (with CRESCUE=1): on a trigger,
  reject the step and take 2dt coupled steps with the fluid subcycled
  at 2x (fluid dt unchanged = the s=2 interpolated closure) for M
  coupled steps, then resume; episodes counted, cap 5 per second of
  march (loud). rescue.rs: coarse_step / attempt_with generalisation;
  march loop is now a while loop (a coarse step consumes two indices,
  the series carries a linear midpoint). VERDICT: refuted 2/2 — the
  coarse steps themselves cannot close once the state is 10x wild;
  both rungs act too late (the kinematic trigger is the limitation).
- RTX_{prefix}_INCTRACE=<csv>: reporting-only per-step dump (step, t,
  predictor increment, tol_step, passes, residual, stalled, tip jump)
  — rung A''s calibration data (healthy anchor vs death).

Verified: the FSI2 committed default digit-identical knob-off after
each change (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/turek_hron_fsi3.rs

@@ -134,6 +134,8 @@ fn fsi3_added_mass_flag() {
             trace_steps: 0,
             trace_from: usize::MAX,
             coupling_rescue: false,
+            coarse_episode: 0,
+            inc_trace: None,
             // 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