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/socket/client/client.go

@@ -0,0 +1,32 @@
+package main
+
+import (
+	"unsafe"
+
+	"github.com/goplus/lib/c"
+	"github.com/goplus/lib/c/net"
+	"github.com/goplus/lib/c/os"
+)
+
+func main() {
+	sockfd := net.Socket(net.AF_INET, net.SOCK_STREAM, 0)
+	msg := c.Str("Hello, World!")
+	defer os.Close(sockfd)
+
+	server := net.GetHostByName(c.Str("localhost"))
+	if server == nil {
+		c.Perror(c.Str("hostname get error"))
+		return
+	}
+
+	servAddr := &net.SockaddrIn{}
+	servAddr.Family = net.AF_INET
+	servAddr.Port = net.Htons(uint16(1234))
+	c.Memcpy(unsafe.Pointer(&servAddr.Addr.Addr), unsafe.Pointer(*server.AddrList), uintptr(server.Length))
+
+	if res := net.Connect(sockfd, (*net.SockAddr)(unsafe.Pointer(servAddr)), c.Uint(16)); res < 0 {
+		c.Perror(c.Str("connect error"))
+		return
+	}
+	os.Write(sockfd, unsafe.Pointer(msg), c.Strlen(msg))
+}