docs: S0 draft — align commit wording, name prescribed-study observable

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/moving_bodies.md

@@ -181,9 +181,9 @@ experiment above.
   moving body exists; swept band touching domain boundaries, other bodies,
   sensors or force regions; `set_body` offset outside the declared box,
   per-step move ≥ 1 cell, speed above the declared bound.
-- Host state is committed only after a successful upload (same contract as Jet
-  amplitude updates); an uncertain device-write failure makes the simulation
-  unusable.
+- Host motion state is committed only after the upload *and* the following
+  synchronization of the step group succeed (section 4); any failure in
+  between makes the simulation unusable and no host/device match is reported.
 - Device-side soft diagnostics (refill fallbacks) are counters, not errors.
 
 ## 8. Coupling and units (CANDIDATE)
@@ -222,7 +222,8 @@ protocol and physical reference uncertainty must be fixed before adoption, and
 none may be weakened after seeing runs):
 normalised momentum-ledger residual FP32 ≤ 5e-6 (FP64 oracle ≤ 2e-13);
 10-cycle relative mass drift ≤ 1e-4; prescribed-motion grid study D = 40/80/160
-with final-step amplitude change ≤ 2 % and phase ≤ 2°; enlarged domain
+with change in the force-response amplitude (observed quantity and norm to be
+named before adoption) ≤ 2 % and phase ≤ 2°; enlarged domain
 amplitude change ≤ 1 %; matched-frame force RMS ≤ 2 % and improving with
 refinement; VIV refinement amplitude ≤ 2 %, frequency ≤ 1 %, five consecutive
 cycle amplitudes within 1 %.