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
+}