Wrap PBI status flags in accessors

gfPreBattleInterfaceActive and gfUsePersistantPBI are now file-static in
PreBattle Interface.cpp, reached only through Is/Set accessors declared in
the header. No behavior change: getters return BOOLEAN so ==TRUE/==FALSE
sites are untouched semantically.

Verify: grep 'gfPreBattleInterfaceActive\|gfUsePersistantPBI' across the tree
hits only the two static definitions, the four accessor bodies (all in
PreBattle Interface.cpp), and two commented-out Asserts in Strategic
Movement.cpp. Every other former site now calls an accessor. Build: JA2.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Strategic/strategicmap.cpp

@@ -178,8 +178,6 @@ INT8 gbRegToElitePromotions = 0;
 INT8 gbMilitiaPromotions = 0;
 
 
-extern BOOLEAN gfUsePersistantPBI;
-
 BOOLEAN		gfUseAlternateMap = FALSE;
 // whether or not we have found Orta yet
 BOOLEAN fFoundOrta = FALSE;
@@ -6527,7 +6525,7 @@ BOOLEAN HandlePotentialBringUpAutoresolveToFinishBattle( int pSectorX, int pSect
 							gfEnteringMapScreen = TRUE;
 							gfEnteringMapScreenToEnterPreBattleInterface = TRUE;
 							gfAutomaticallyStartAutoResolve = TRUE;
-							gfUsePersistantPBI = FALSE;
+							SetPersistantPBI( FALSE );
 							gubPBSectorX = (UINT8)pSectorX;
 							gubPBSectorY = (UINT8)pSectorY;
 							gubPBSectorZ = (UINT8)pSectorZ;