Mech phase 4: SetMappingSubsystem + MechControlsMapper ctor -- mech links as viewpoint

SetMappingSubsystem installs the control mapper into roster slot 0 (the streamed
control-mapping binds DirectMappings to subsystemID 0). MechControlsMapper ctor
constructs (per-frame InterpretControls deferred to phase 5). With RIO controls,
MakeViewpointEntity now builds the MechRIOMapper, installs it, and gets PAST the
controls check -- the mech constructs AND links as the viewpoint entity. Boot now
advances into the mission-startup / control-binding path (next: a real NULL-deref
there, deeper than the staged Fails). BT: 42 ok.

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

restoration/source410/BT/MECH.CPP

@@ -301,7 +301,20 @@ Mech::~Mech()
 }
 
 void
-	Mech::SetMappingSubsystem(Subsystem *)
+	Mech::SetMappingSubsystem(Subsystem *subsystem)
 {
-	Fail("Mech::SetMappingSubsystem -- mech.cpp not yet reconstructed");
+	Check(this);
+	Check_Pointer(subsystemArray);
+
+	//
+	// The control mapper lives in roster slot 0 (the streamed control-mapping
+	// resource binds its DirectMappings to subsystemID 0, so it must resolve
+	// there via Entity::GetSimulation(0)).  On a re-spawn, drop the old one.
+	//
+	if (subsystemArray[0] != NULL)
+	{
+		Unregister_Object(subsystemArray[0]);
+		delete subsystemArray[0];
+	}
+	subsystemArray[0] = subsystem;
 }

restoration/source410/BT/MECHMPPR.CPP

@@ -44,7 +44,19 @@ MechControlsMapper::MechControlsMapper(
 		shared_data
 	)
 {
-	Fail("MechControlsMapper -- mechmppr.cpp not yet reconstructed");
+	Check(owner);
+	//
+	// The mapper is the Mech's active control Performance; per-frame control
+	// interpretation (InterpretControls) is reconstructed with the mech2/3/4
+	// simulation (phase 5).  Prime the published control state to neutral.
+	//
+	{
+		for (int i = 0; i < 64; ++i)
+		{
+			reserved[i] = 0;
+		}
+	}
+	Check_Fpu();
 }
 
 MechControlsMapper::~MechControlsMapper()