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/genints/genints.go

@@ -0,0 +1,63 @@
+package main
+
+import (
+	"github.com/goplus/lib/c"
+)
+
+type generator struct {
+	val c.Int
+}
+
+func (g *generator) next() c.Int {
+	g.val++
+	return g.val
+}
+
+func genInts(n int, gen func() c.Int) []c.Int {
+	a := make([]c.Int, n)
+	for i := range a {
+		a[i] = gen()
+	}
+	return a
+}
+
+func main() {
+	// generate 5 random integers
+	for _, v := range genInts(5, c.Rand) {
+		c.Printf(c.Str("%d\n"), v)
+	}
+
+	// generate 5 integers, each is double of the previous one
+	initVal := c.Int(1)
+	ints := genInts(5, func() c.Int {
+		initVal *= 2
+		return initVal
+	})
+	for _, v := range ints {
+		c.Printf(c.Str("%d\n"), v)
+	}
+
+	// generate 5 integers, each is incremented by 1
+	g := &generator{val: 1}
+	for _, v := range genInts(5, g.next) {
+		c.Printf(c.Str("%d\n"), v)
+	}
+}
+
+/* Posible output:
+16807
+282475249
+1622650073
+984943658
+1144108930
+2
+4
+8
+16
+32
+2
+3
+4
+5
+6
+*/