Bump version to v5.1.5, remove tracked build artifacts

- Bump version 5.1.4 → 5.1.5 across all source, CI, and packaging files
- Remove 157 build artifacts (build/*.o, release/*.dll, etc.) from git tracking
  These were added before .gitignore existed and caused cross-platform CI failures

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/clientversion.h

@@ -8,7 +8,7 @@
 // These need to be macros, as version.cpp's and triangles-qt.rc's voodoo requires it
 #define CLIENT_VERSION_MAJOR       5
 #define CLIENT_VERSION_MINOR       1
-#define CLIENT_VERSION_REVISION    4
+#define CLIENT_VERSION_REVISION    5
 #define CLIENT_VERSION_BUILD       0
 
 // Converts the parameter X to a string after macro replacement on X has been performed.

src/version.h

@@ -53,7 +53,7 @@ static const int MEMPOOL_GD_VERSION = 60002;
 
 #define DISPLAY_VERSION_MAJOR       5
 #define DISPLAY_VERSION_MINOR       1
-#define DISPLAY_VERSION_REVISION    4
+#define DISPLAY_VERSION_REVISION    5
 #define DISPLAY_VERSION_BUILD       0
 
 #endif