fsi2 overset harness: RTX_FSI2O_CASE=FSI3 (the FSI3 constants with FEATFLOW level-4 references in the summary), RTX_FSI2O_SAVE_FROM (instants only from a time on — a dense last period for the viewer)

Co-Authored-By: Claude Fable 5.1 <[email protected]>
Claude-Session: https://claude.ai/code/session_01LzcjQX7tvgn87CQCyg9Cfr

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

@@ -283,6 +283,10 @@ pub fn run_march_overset(case: BenchmarkCase, config: &OversetMarchConfig) -> Ov
     });
     let (t_fluid, t_structure) = (std::cell::Cell::new(0.0_f64), std::cell::Cell::new(0.0_f64));
     let prev_area = std::cell::Cell::new(fluid.borrow().shared.read().unwrap().area());
+    let save_from: f64 = std::env::var("RTX_FSI2O_SAVE_FROM")
+        .ok()
+        .and_then(|v| v.parse().ok())
+        .unwrap_or(0.0);
     let save_every: usize = std::env::var("RTX_FSI2O_SAVE_EVERY")
         .ok()
         .and_then(|v| v.parse().ok())
@@ -418,7 +422,9 @@ pub fn run_march_overset(case: BenchmarkCase, config: &OversetMarchConfig) -> Ov
         fluid.borrow_mut().commit_base();
         // `RTX_FSI2O_SAVE_EVERY=N` (+ `RTX_FSI2O_SAVE`): the composite at
         // every N-th committed step, for the offline chain audit.
-        if save_every > 0 && (step + 1) % save_every == 0 {
+        // `RTX_FSI2O_SAVE_FROM=t` (0): instants only from t on — a dense
+        // last period for the viewer without a gigabyte of the onset.
+        if save_every > 0 && (step + 1) % save_every == 0 && t_now >= save_from {
             if let Ok(dir) = std::env::var("RTX_FSI2O_SAVE") {
                 let d_now = extract(&flag_state);
                 let dd_now = extract_velocity(&flag_state);