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/readdir/main.go

@@ -0,0 +1,29 @@
+package main
+
+import (
+	"fmt"
+	"os"
+)
+
+func main() {
+	entries, err := os.ReadDir("../")
+	if err != nil {
+		panic(err)
+	}
+	if len(entries) == 0 {
+		panic("No files found")
+	}
+	var check int
+	for _, e := range entries {
+		fmt.Printf("%s isDir[%t]\n", e.Name(), e.IsDir())
+		if !e.IsDir() {
+			switch e.Name() {
+			case "go.sum", "go.mod":
+				check++
+			}
+		}
+	}
+	if check != 2 {
+		panic("Bad readdir entries go.mod/go.sum")
+	}
+}