vehicleSubSystems: reconstruct the stubbed connection helpers from decomp

Replace the return-0 stubs with the real functions:
- ResolveLink (FUN_00417ab4) = SubsystemConnection::Resolve (two-level deref of the
  plug's bound link); an unbound plug resolves to 0, the authentic no-source branch.
- FindLinkedSubsystem (FUN_0041bf44) = GetAttributePointer(index) via the engine
  Simulation attribute-index system (the cooling-loop master at slot 3).
- IsGeneratorDerived (FUN_0041a1a4) = the real IsDerivedFrom(PoweredSubsystem
  ClassDerivations 0x50f4bc) via a new powersub.cpp bridge BTIsPoweredSubsystem
  (shared with the aux-screen filter) -> the generator stateLamp now builds.

So the cooling/power/voltage connections resolve real data when the plug is bound,
instead of always reading 0.  Build clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

game/reconstructed/btl4gau2.cpp

@@ -124,13 +124,21 @@ static void *AttributePointerOf(void *subsystem, const char *name)
 }
 
 //
-// FUN_00417ab4 -- follow a SharedData/plug link to the resolved subsystem (voltage
-// source / cooling master).  The reconstruction stubs this everywhere (see
-// emitter.cpp) because the roster plug-binding pass is not yet reconstructed;
-// matched here so the cluster connections read the safe "no source" branch
-// (*destination = 0) until the plug binding lands.  BEST-EFFORT (marked).
+// FUN_00417ab4 -- SubsystemConnection::Resolve: follow the plug's bound link to the
+// resolved subsystem.  The link is `*(plug+8)`; the resolved subsystem is
+// `*(link+8)`.  An unbound plug (link == 0) resolves to 0 -- the authentic
+// behaviour when a subsystem has no attached voltage source / cooling master (the
+// connection then reads its "no source" branch, *destination = 0).
 //
-static void *ResolveLink(void *plug)			{ (void)plug; return 0; }
+static void *ResolveLink(void *plug)
+{
+	if (plug == 0)
+		return 0;
+	void *link = *(void **)((char *)plug + 8);
+	if (link == 0)
+		return 0;
+	return *(void **)((char *)link + 8);
+}
 
 //
 // Bridge (defined in powersub.cpp) that reads the engineering-screen assignment
@@ -143,20 +151,31 @@ static void *ResolveLink(void *plug)			{ (void)plug; return 0; }
 extern bool BTGetSubsystemAuxScreen(Subsystem *sub, int *screen, int *placement, char *label64);
 
 //
-// FUN_0041bf44 -- fetch the subsystem linked at slot N (used by the cooling-loop
-// lamp to find its cooling master).  Stubbed to match ResolveLink above.
+// FUN_0041bf44 -- GetAttributePointer(index): a pointer to the subsystem's Nth
+// attribute member (the cooling-loop lamp reaches its cooling-master link at
+// attribute slot 3), or 0 if the index is out of range.  The engine exposes this
+// via Simulation::GetAttributePointer(AttributeID); a miss returns &NullAttribute,
+// normalised to 0.
 //
-static void *FindLinkedSubsystem(void *subsystem, int slot) { (void)subsystem; (void)slot; return 0; }
+static void *FindLinkedSubsystem(void *subsystem, int slot)
+{
+	if (subsystem == 0)
+		return 0;
+	void *p = ((Simulation *)subsystem)->GetAttributePointer((Simulation::AttributeID)slot);
+	if (p == (void *)&Simulation::NullAttribute)
+		return 0;
+	return p;
+}
 
 //
-// FUN_0041a1a4 IsDerivedFrom(0x50f4bc == Generator's ClassDerivations).  Gates the
-// SubsystemCluster's generator stateLamp.  BEST-EFFORT: the Generator
-// ClassDerivations object is owned by the (separately-reconstructed) gnrator TU
-// which must not be pulled into this gauge TU (stub-collision, see btl4gaug.hpp),
-// so the check is stubbed to False -- the generator-only stateLamp is simply not
-// built.  Lands with a shared IsDerivedFrom accessor.
+// FUN_0041a1a4 IsDerivedFrom(0x50f4bc == PoweredSubsystem's ClassDerivations).
+// Gates the SubsystemCluster's generator stateLamp.  Every subsystem that reaches
+// a SubsystemCluster passed the vehicleSubSystems type filter (BTGetSubsystemAuxScreen
+// -> the SAME IsDerivedFrom(PoweredSubsystem) check), so this is always True here;
+// the real check is done in powersub.cpp via BTIsPoweredSubsystem for robustness.
 //
-static Logical IsGeneratorDerived(Subsystem *) { return False; }
+extern bool BTIsPoweredSubsystem(Subsystem *sub);
+static Logical IsGeneratorDerived(Subsystem *sub) { return BTIsPoweredSubsystem(sub) ? True : False; }
 
 
 //###########################################################################

game/reconstructed/powersub.cpp

@@ -1269,11 +1269,16 @@ Subsystem *CreatePoweredSubsystem(Mech *owner, int id, void *seg)
 // non-PoweredSubsystem-derived subsystems (== the FUN_0041a1a4 / 0x50f4bc type
 // filter the vehicleSubSystems Make applies before dispatch).
 //
+// FUN_0041a1a4(sub->classDerivations, 0x50f4bc) -- is the subsystem PoweredSubsystem-
+// derived?  (The vehicleSubSystems type filter + the generator stateLamp gate.)
+bool BTIsPoweredSubsystem(Subsystem *sub)
+{
+	return sub != NULL && sub->IsDerivedFrom(*PoweredSubsystem::GetClassDerivations());
+}
+
 bool BTGetSubsystemAuxScreen(Subsystem *sub, int *screen, int *placement, char *label64)
 {
-	if (sub == NULL)
-		return false;
-	if (!sub->IsDerivedFrom(*PoweredSubsystem::GetClassDerivations()))	// FUN_0041a1a4(...,0x50f4bc)
+	if (!BTIsPoweredSubsystem(sub))						// FUN_0041a1a4(...,0x50f4bc)
 		return false;
 	PoweredSubsystem *ps = (PoweredSubsystem *)sub;
 	if (screen != NULL)    *screen    = ps->auxScreenNumber;			// +0x104