CROUCH complete: full cycle + MP replication verified

The WIP's 'pose does not hold' was a chain of bench-instrument errors, not a
code bug: every capture ran in COCKPIT view (the pilot cannot see their own
legs; the eye-height residual masked as reversion).  Joint probes prove the
park holds indefinitely (knee 1.138, root -2.219 steady); the 2-node bench
shows the observer's replicant fully crouched and held (duckmpA_031 -- the
type-3 state record carries it with zero new replication code); the second
scripted press (new BT_BTNTEST2 env) verifies RISE -> standing zeros.
MP button delivery confirmed mode-mask-clean (the one miss was round-start
jitter).  Diags added, all BT_DUCK_LOG-gated: SetLegAnimation re-arm tracer,
1 Hz joint probe, RIO press mode-mask, BT_TREE_LOG topology dump, and the
squat-park log.  RESIDUAL filed: pilot's own cockpit eye does not ride the
root drop (DPLEyeRenderable chain composition; cosmetic).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

engine/MUNGA_L4/L4CTRL.cpp

@@ -2624,6 +2624,13 @@ void
 		Verify(rio_event.Data.Unit < ButtonCount);
 		temp = rio_event.Data.Unit + 1;
 
+		// DIAG (BT_DUCK_LOG): the mode mask a press dispatches under -- the
+		// crouch-button (0x13) MP-delivery hunt (mode-gated streamed rows).
+		if (getenv("BT_DUCK_LOG"))
+			DEBUG_STREAM << "[duck] RIO press unit=0x" << std::hex
+				<< (int)rio_event.Data.Unit << " modeMask=0x"
+				<< (unsigned)mode_mask << std::dec << "\n" << std::flush;
+
 		Check(&buttonGroup[rio_event.Data.Unit]);
 		buttonGroup[rio_event.Data.Unit].Update(&temp, mode_mask);
 		//----------------------------------------