test(rtx-fsi): RTX_FSI{2,3}_UMEAN inflow override for the TWIN-1 parameter sweep

case_from_env applies the knob over the case's benchmark inflow; unset,
the same f64 flows and both committed defaults are verified
digit-identical in vivo (physics lines diff-clean vs same-day
baselines; only wall-split timing percentages moved). Physics bands in
both tests are guarded to benchmark inflow — an off-benchmark march
(the sweep) asserts machinery invariants only; its bands live in the
TWIN-1 composition harness (omni-cortex
docs/twin_composition_campaign.md). The override prints loudly so
sweep logs are self-describing.

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

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

@@ -175,7 +175,8 @@ fn fsi2_flapping_flag() {
         smooth_in_h,
         ..
     } = config;
-    let result = run_march(FSI2, &config);
+    let case = fsi2_harness::case_from_env("FSI2", FSI2);
+    let result = run_march(case, &config);
     let w = result.window(3.0);
 
     println!(
@@ -250,9 +251,16 @@ fn fsi2_flapping_flag() {
     // 2.23 Hz. If a change moves any of these numbers, that is a finding
     // either way and must be loud. Smoothing changes the load path and
     // is pinned nowhere (measured to change nothing that matters).
-    let default_coupling = smooth_in_h == 0.0 && config.coupler == "aitken";
-    let mode2_coupling = smooth_in_h == 0.0 && config.coupler == "iqn" && subcycle == 2;
-    let s1_coupling = smooth_in_h == 0.0 && config.coupler == "iqn" && subcycle == 1;
+    // Every physics band below was measured at the BENCHMARK inflow; an
+    // overridden u_mean (the TWIN-1 sweep) pins nothing here — its bands
+    // live in the campaign's composition harness. The machinery
+    // invariants above stay asserted at every inflow.
+    let benchmark_inflow = case.u_mean.to_bits() == FSI2.u_mean.to_bits();
+    let default_coupling = benchmark_inflow && smooth_in_h == 0.0 && config.coupler == "aitken";
+    let mode2_coupling =
+        benchmark_inflow && smooth_in_h == 0.0 && config.coupler == "iqn" && subcycle == 2;
+    let s1_coupling =
+        benchmark_inflow && smooth_in_h == 0.0 && config.coupler == "iqn" && subcycle == 1;
     if default_coupling
         && ny == 62
         && flag_nx == 35