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/qsort/qsort.go

@@ -0,0 +1,25 @@
+package main
+
+import (
+	"unsafe"
+
+	"github.com/goplus/lib/c"
+)
+
+func main() {
+	a := [...]int{100, 8, 23, 2, 7}
+	c.Qsort(c.Pointer(&a), 5, unsafe.Sizeof(0), func(a, b c.Pointer) c.Int {
+		return c.Int(*(*int)(a) - *(*int)(b))
+	})
+	for _, v := range a {
+		c.Printf(c.Str("%d\n"), v)
+	}
+}
+
+/* Expected output:
+2
+7
+8
+23
+100
+*/