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/netdbdemo/netdb.go

@@ -0,0 +1,20 @@
+package main
+
+import (
+	"github.com/goplus/lib/c"
+	"github.com/goplus/lib/c/net"
+)
+
+func main() {
+	var hints net.AddrInfo
+	hints.Family = net.AF_UNSPEC
+	hints.SockType = net.SOCK_STREAM
+
+	host := "httpbin.org"
+	port := "80"
+
+	var result *net.AddrInfo
+	c.Printf(c.Str("%d\n"), net.Getaddrinfo(c.Str(host), c.Str(port), &hints, &result))
+
+	c.Printf(c.Str("%d\n"), net.Freeaddrinfo(result))
+}