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/thread/thd.go

@@ -0,0 +1,25 @@
+package main
+
+import (
+	"github.com/goplus/lib/c"
+	"github.com/goplus/lib/c/pthread"
+)
+
+var key pthread.Key
+
+func main() {
+	key.Create(nil)
+	key.Set(c.Pointer(c.Str("main value\n")))
+
+	var thd pthread.Thread
+	pthread.Create(&thd, nil, func(arg c.Pointer) c.Pointer {
+		key.Set(c.Pointer(c.Str("thread value\n")))
+		c.Printf(c.Str("Hello, thread\nTLS: %s"), key.Get())
+		return c.Pointer(c.Str("Back to main\n"))
+	}, nil)
+
+	var retval c.Pointer
+	pthread.Join(thd, &retval)
+
+	c.Printf(c.Str("%sTLS: %s"), retval, key.Get())
+}