Fix circular includes and include cleanup (#3)

* Fix circular includes and include cleanup

Fixed a bug where other *.h header files would include extension.h, which itself
was already included in those same headers, creating a circular dependency cycle.
This caused build errors and made compilation unpredictable.

To resolve this:
- Moved implementation-only includes to .cpp files where they are actually needed.
- Added forward declarations where possible to avoid pulling in unnecessary headers.
- Ensured extension.h only contains declarations that require full type definitions.

This improves build stability and compilation time.

* Clean up includes and fix build errors

- Replaced full includes with forward declarations where possible
- Moved implementation-only includes from .h to .cpp files
- Added missing includes for stdint.h and Steam interfaces
- This significantly reduces compilation time and prevents future dependency issues

* Switch to SourceMod SDK smsdk_ext instead of local copy

Extension/swforwards.h

@@ -17,9 +17,13 @@
 */
 
 #pragma once
-#include "isteamgameserver.h"
-#include "steam_gameserver.h"
-#include "smsdk_ext.h"
+
+#include <steam_gameserver.h>
+
+namespace SourceMod
+{
+	class IForward;
+}
 
 typedef uint32_t Account_t;
 
@@ -41,10 +45,10 @@ class SteamWorksForwards
 		STEAM_GAMESERVER_CALLBACK(SteamWorksForwards, OnGroupStatusResult, GSClientGroupStatus_t, m_CallbackGroupStatus);
 	
 	private:
-		IForward *pFOVC;	/* Forward On Validate Client */
-		IForward *pFOVC_Old;	/* OLD Forward On Validate Client */
-		IForward *pFOSSC;	/* Forward On Steam Servers Connected */
-		IForward *pFOSSCF;	/* Forward On Steam Servers Connect Failure */
-		IForward *pFOSSD;	/* Forward On Steam Servers Disconnected */
-		IForward *pFOCGS;	/* Forward On Client Group Status */
+		SourceMod::IForward *pFOVC;	/* Forward On Validate Client */
+		SourceMod::IForward *pFOVC_Old;	/* OLD Forward On Validate Client */
+		SourceMod::IForward *pFOSSC;	/* Forward On Steam Servers Connected */
+		SourceMod::IForward *pFOSSCF;	/* Forward On Steam Servers Connect Failure */
+		SourceMod::IForward *pFOSSD;	/* Forward On Steam Servers Disconnected */
+		SourceMod::IForward *pFOCGS;	/* Forward On Client Group Status */
 };