Implement multi-platform build system with pure Go support

- Introduced a comprehensive build system that supports multiple platforms (Linux, macOS, Windows, Android) using pure Go builds (`CGO_ENABLED=0`).
- Updated all build and test scripts to ensure compatibility with the new purego approach, allowing for dynamic loading of `libsecp256k1` at runtime.
- Added detailed documentation on the build process, platform detection, and deployment options.
- Enhanced CI/CD workflows to automate builds for all supported platforms and include necessary libraries in releases.
- Updated `.gitignore` to exclude build output files.
- Created new documentation files for deployment and multi-platform build summaries.

scripts/test.sh

@@ -1,4 +1,11 @@
 #!/usr/bin/env bash
+# Pure Go build with purego - no CGO needed
+# libsecp256k1 is loaded dynamically at runtime if available
+export CGO_ENABLED=0
+if [ -f "pkg/crypto/p8k/libsecp256k1.so" ]; then
+    export LD_LIBRARY_PATH="${LD_LIBRARY_PATH:+$LD_LIBRARY_PATH:}$(pwd)/pkg/crypto/p8k"
+fi
+
 go mod tidy
 go test ./...
 cd pkg/crypto