feat: Implement platform-specific hosts file path retrieval

- Created a new `platform` package to handle OS-specific hosts file paths.
- Added implementations for macOS, Linux, and Windows to retrieve the hosts file path.
- Introduced tests for the `GetHostsFilePath` function to ensure correct behavior across platforms.

feat: Add DNS resolver for IP lookups

- Implemented a `resolver` package for performing DNS lookups.
- Created a `DNSResolver` struct with a `LookupIP` method to handle A-record lookups and CNAME resolution.
- Added comprehensive tests for various DNS scenarios, including CNAME chains and error handling.

chore: Refactor project structure and update dependencies

- Restructured the project to follow Go's standard package layout.
- Updated `go.mod` to Go 1.20 and added `golang.org/x/net` dependency.
- Removed old source files and ensured a clean build with all tests passing.

platform/platform_test.go

@@ -0,0 +1,56 @@
+package platform_test
+
+import (
+	"os"
+	"strings"
+	"testing"
+
+	"dns-helper/platform"
+)
+
+// T007-1: GetHostsFilePath returns a non-empty path with no error on the current OS.
+func TestGetHostsFilePathReturnsPath(t *testing.T) {
+	path, err := platform.GetHostsFilePath()
+	if err != nil {
+		t.Fatalf("GetHostsFilePath returned unexpected error: %v", err)
+	}
+	if path == "" {
+		t.Error("GetHostsFilePath returned empty path")
+	}
+}
+
+// T007-2: The returned path exists on the filesystem.
+func TestGetHostsFilePathExists(t *testing.T) {
+	path, err := platform.GetHostsFilePath()
+	if err != nil {
+		t.Fatalf("GetHostsFilePath returned unexpected error: %v", err)
+	}
+	if _, err := os.Stat(path); err != nil {
+		t.Errorf("hosts file path %q does not exist or cannot be accessed: %v", path, err)
+	}
+}
+
+// T024-1: GetHostsFilePath returns a path that is an absolute path (contains a
+// directory separator), ensuring the error messages that embed the path are descriptive.
+func TestGetHostsFilePathIsAbsolute(t *testing.T) {
+	path, err := platform.GetHostsFilePath()
+	if err != nil {
+		t.Fatalf("GetHostsFilePath returned unexpected error: %v", err)
+	}
+	if !strings.ContainsAny(path, `/\`) {
+		t.Errorf("expected an absolute path containing a directory separator, got %q", path)
+	}
+}
+
+// T024-2: GetHostsFilePath function signature returns (string, error) — process
+// termination (os.Exit, log.Fatal, term()) is absent from the platform package.
+// This is verified statically by T023 grep/vet; the runtime counterpart is that the
+// function can be called without panicking or exiting.
+func TestGetHostsFilePathDoesNotPanic(t *testing.T) {
+	defer func() {
+		if r := recover(); r != nil {
+			t.Errorf("GetHostsFilePath panicked: %v", r)
+		}
+	}()
+	platform.GetHostsFilePath() //nolint:errcheck // return values checked in other tests
+}