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/async/async/async.go

@@ -0,0 +1,31 @@
+package async
+
+import (
+	_ "unsafe"
+)
+
+type Void = [0]byte
+
+type Future[T any] interface {
+	Then(cb func(T))
+}
+
+type future[T any] struct {
+	cb func(func(T))
+}
+
+func (f *future[T]) Then(cb func(T)) {
+	f.cb(cb)
+}
+
+func Async[T any](fn func(func(T))) Future[T] {
+	return &future[T]{fn}
+}
+
+func Run[T any](future Future[T]) T {
+	var ret T
+	future.Then(func(v T) {
+		ret = v
+	})
+	return ret
+}

_demo/go/async/main.go

@@ -0,0 +1,23 @@
+package main
+
+import (
+	"time"
+
+	"github.com/goplus/llgo/_demo/go/async/async"
+	"github.com/goplus/llgo/_demo/go/async/timeout"
+)
+
+func Sleep(i int, d time.Duration) async.Future[int] {
+	return async.Async(func(resolve func(int)) {
+		timeout.Timeout(d).Then(func(async.Void) {
+			resolve(i)
+		})
+	})
+}
+
+func main() {
+	async.Run(async.Async(func(resolve func(async.Void)) {
+		println("read file")
+		defer resolve(async.Void{})
+	}))
+}

_demo/go/async/timeout/timeout.go

@@ -0,0 +1,16 @@
+package timeout
+
+import (
+	"time"
+
+	"github.com/goplus/llgo/_demo/go/async/async"
+)
+
+func Timeout(d time.Duration) async.Future[async.Void] {
+	return async.Async(func(resolve func(async.Void)) {
+		go func() {
+			time.Sleep(d)
+			resolve(async.Void{})
+		}()
+	})
+}