4.10 reconstruction: real spawn factory path -> Mech ctor frontier

BTPlayer::CreatePlayerVehicle reconstructed (faithful; BT411's BT_SPAWN_XZ/
BT_SPAWN_ENEMY bring-up scaffolding dropped): builds the Mech::MakeMessage from
the mission game-model resource and hands it to MakeAndLinkViewpointEntity.
Mech::Make = new Mech(creation_message); a staged Mech ctor chains the real
JointedMover base (so the model skeleton/segments stream from BTL4.RES) then
Fails.

This validates the entire spawn factory chain live: CreatePlayerVehicle ->
MakeAndLinkViewpointEntity -> registry -> Mech::Make -> new Mech -> JointedMover
base ctor (streams the mech model) -> halts at the Mech ctor (mech.cpp:66).

The Mech ctor (@004a1674, 5690 bytes -- the largest function in the game) plus
the subsystem roster is the next MAJOR milestone, not a brick: it needs the full
1995 Mech member layout derived first (MECH.HPP is still reserved[331]; BT411's
decomp uses raw offset pokes that can't compile fresh), the subsystem class
hierarchy (only GAUSS/PPC/SENSOR survive in the 4.10 archive; the rest are the
measured "917 missing functions"), and the segment-table walk. Plan + scope in
MECH.NOTES.md.

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

restoration/source410/BT/MECH.CPP

@@ -43,10 +43,31 @@ Mech::SharedData
 //#############################################################################
 //
 Mech*
-	Mech::Make(MakeMessage *)
+	Mech::Make(MakeMessage *creation_message)
+{
+	return new Mech(creation_message);
+}
+
+//
+//#############################################################################
+// Mech ctor -- the heart of the entity (walks the model segment table and
+// instantiates the full subsystem roster: power, heat, weapons, actuators,
+// controls, tech, damage zones).  This is the largest single function in the
+// game and the current reconstruction frontier; see MECH.NOTES.md.  Chains to
+// the JointedMover base so the skeleton/segments stream before it Fails.
+//#############################################################################
+//
+Mech::Mech(
+	MakeMessage	*creation_message,
+	SharedData	&shared_data
+):
+	JointedMover(creation_message, shared_data)
+{
+	Fail("Mech::Mech -- mech.cpp not yet reconstructed");
+}
+
+Mech::~Mech()
 {
-	Fail("Mech::Make -- mech.cpp not yet reconstructed");
-	return NULL;
 }
 
 void