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/c/cppintf/cppintf.go

@@ -0,0 +1,38 @@
+package main
+
+import (
+	"github.com/goplus/lib/c"
+	"github.com/goplus/lib/c/math"
+	"github.com/goplus/llgo/_demo/c/cppintf/foo"
+)
+
+type Bar struct {
+	foo.Callback
+	a c.Int
+}
+
+func NewBar(a c.Int) *Bar {
+	return &Bar{
+		Callback: foo.Callback{
+			Vptr: &foo.CallbackVtbl{
+				Val:  c.Func((*Bar).getA),
+				Calc: c.Func((*Bar).sqrt),
+			},
+		},
+		a: a,
+	}
+}
+
+func (p *Bar) getA() c.Int {
+	return p.a
+}
+
+func (p *Bar) sqrt(v float64) float64 {
+	return math.Sqrt(v)
+}
+
+func main() {
+	bar := NewBar(1)
+	foo.F(&bar.Callback)
+	foo.G(&bar.Callback)
+}