Remove build directory from version control and add to .gitignore

The build directory contains temporary build artifacts that are
regenerated by build_xcframework.sh and upgrade_framework.sh scripts.
These files should not be tracked in version control.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

.gitignore

@@ -0,0 +1,8 @@
+# Build output
+build/
+
+# macOS
+.DS_Store
+
+# Backup directories
+backup_*/