reorganize: consolidate demo directories

- Consolidate _demo, _pydemo, _embdemo into single _demo directory structure
- Organize demos by language: _demo/{go,py,c,embed}/
- Categorize demos based on imports:
- Python library demos (py imports) → _demo/py/
- C/C++ library demos (c/cpp imports) → _demo/c/
- Go-specific demos → _demo/go/
- Embedded demos → _demo/embed/
- Move C-related demos (asm*, cabi*, cgo*, linkname, targetsbuild) from go/ to c/
- Update all path references in README.md and GitHub workflows
- Improve demo organization and navigation as requested in #1256

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

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

_demo/go/mkdirdemo/mkdir.go

@@ -0,0 +1,34 @@
+package main
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+)
+
+func main() {
+	tempDir, err := os.MkdirTemp("", "example*")
+	if err != nil {
+		fmt.Println("Failed to create temp directory:", err)
+		return
+	}
+	defer os.Remove(tempDir)
+	fmt.Println("Temp directory:", tempDir)
+
+	tempFile, err := os.CreateTemp("", "example*.txt")
+	if err != nil {
+		fmt.Println("Failed to create temp file:", err)
+		return
+	}
+	defer tempFile.Close()
+	defer os.Remove(tempFile.Name())
+	fmt.Println("Temp file:", tempFile.Name())
+
+	nestedDir := filepath.Join("nested", "directory")
+	err = os.MkdirAll(nestedDir, 0755)
+	if err != nil {
+		fmt.Println("Failed to create nested directory:", err)
+		return
+	}
+	fmt.Println("Nest directory:", nestedDir)
+}