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/crand/rand.go

@@ -0,0 +1,26 @@
+package main
+
+import (
+	"fmt"
+
+	"github.com/goplus/lib/c"
+	"github.com/goplus/lib/c/math/rand"
+	"github.com/goplus/lib/c/time"
+)
+
+func fastrand64() uint64 {
+	v1 := uint64(rand.Random())
+	v2 := uint64(rand.Random())
+	return v1 ^ (v2 << 32)
+}
+
+func main() {
+	rand.Srand(c.Uint(time.Time(nil)))
+	fmt.Printf("%x\n", rand.Rand())
+	fmt.Printf("%x\n", rand.Rand())
+
+	rand.Srandom(c.Uint(time.Time(nil)))
+	fmt.Printf("%x\n", rand.Random())
+	fmt.Printf("%x\n", rand.Random())
+	fmt.Printf("%x\n", fastrand64())
+}