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Author	SHA1	Message	Date
Omar SobhandClaude Fable 5	140310b223	rtx-fsi: the noise floor interrogated — smoothing refuted, IQN-ILS lands, tight coupling reopened ... The tenth session ended on "lower the interface noise floor". This builds the levers and measures them, and the measurements overturn the diagnosis: - smooth_tractions: arclength moving average over the wetted surface, area-weighted, smooth normal-similarity factor so corners do not mix and the smoothed load stays continuous in the geometry. Nine unit tests. MEASURED NEGATIVE RESULT: the flip-scan floor is unchanged to 0.2% at radii 1-3h — the flip's load jump is coherent through the fluid field (mask rebuild shifts the pressure around the flipped cell), and a surface average preserves coherent shifts. Default off; the probe pins the attribution so nobody re-reaches for this lever. - IqnIls: interface quasi-Newton with inverse least squares (Degroote 2009) — filtered MGS least squares over secant columns (filter RELATIVE to column norm), cross-step history reuse, per-step set_tolerance. Model-map tests: exact on anisotropic linear maps within dim+2 passes (scalar Aitken provably cannot be), scale invariant, history reuse shortens the next step, stalls at the noise scale instead of diverging (fixture lesson: per-pass noise, not state-dependent noise — the latter has a genuine fixed point). - tests/fsi2_harness/: the FSI2 machinery extracted shared; verified pure code motion (committed release response reproduced to every printed digit). March gains RTX_FSI2_SMOOTH / RTX_FSI2_COUPLER=iqn / RTX_FSI2_REUSE knobs; pinned bands guard the default configuration. - tests/fsi2_interface_noise.rs: the probe. Flip-scan floor at subcycle 8: 3.05e-5 (pinned); smoothing attribution pinned; the cross-subcycle scan recorded but unpinned (the fixed geometry increment's wall-velocity trend, increment/dt_c, swamps the flip signal at small dt_c — a dt_c^2 scaling hypothesis died in that operationalization). THE OPERATIONAL FLOOR — the real release step subiterated at tolerance 1e-9 with residuals traced — converges DEEP at both subcycles: s8 aitken 3.4e-9 / iqn 1.6e-9, s2 both ~6.4e-10 in 5-6 passes. The flip jumps are events at specific geometries, not a floor under every step: the tenth session's subcycle-2 blowup was tolerance mis-budgeting (2e-4 held fixed while dt_c shrank), not an impassable floor. Probe bug found and fixed on the way: stale shared geometry leaked a 4.5e-5 phantom first residual into the first stall run; every measurement now resets the geometry on entry. All 924+17 tests green: lib 44 (was 27), piston 2, curved edge 1, FSI1, the committed FSI2 march (release response identical), the probe. Co-Authored-By: Claude Fable 5 <[email protected]> Claude-Session: https://claude.ai/code/session_01Lnyrw33Lu6rUhW42E9KHwq	2026-08-21 06:42:05 -07:00