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/fcntl/fcntl.go

@@ -0,0 +1,76 @@
+package main
+
+import (
+	"unsafe"
+
+	"github.com/goplus/lib/c"
+	"github.com/goplus/lib/c/os"
+)
+
+func main() {
+
+	filename := c.Str("testfile.txt")
+	data := c.Str("Hello, os!")
+	var buffer [20]c.Char
+
+	// Open a file, O_CREAT|O_WRONLY|O_TRUNC means create, write only, or clear the file
+	fd := os.Open(filename, os.O_CREAT|os.O_WRONLY|os.O_TRUNC, 0644)
+	if fd == -1 {
+		c.Printf(c.Str("open error\n"))
+		return
+	}
+
+	// Writing data to a file
+	bytesWritten := os.Write(fd, c.Pointer(data), c.Strlen(data))
+	if bytesWritten == -1 {
+		c.Printf(c.Str("write error\n"))
+		os.Close(fd)
+		return
+	}
+	c.Printf(c.Str("Written %ld bytes to %s\n"), bytesWritten, filename)
+
+	// Get file status flags
+	flags := os.Fcntl(fd, os.F_GETFL)
+	if flags == -1 {
+		c.Printf(c.Str("os error\n"))
+		os.Close(fd)
+		return
+	}
+	c.Printf(c.Str("File flags: %d\n"), flags)
+
+	// Set the file status flag to non-blocking mode
+	if os.Fcntl(fd, os.F_SETFL, flags|os.O_NONBLOCK) == -1 {
+		c.Printf(c.Str("os error\n"))
+		os.Close(fd)
+		return
+	}
+	c.Printf(c.Str("set file status successfully\n"))
+
+	c.Printf(c.Str("111"))
+	// Close file
+	os.Close(fd)
+
+	// Reopen the file, O_RDONLY means read-only
+	fd = os.Open(filename, os.O_RDONLY)
+	if fd == -1 {
+		c.Printf(c.Str("open error\n"))
+		return
+	}
+
+	// Reading data from a file
+	// &buffer[:][0]
+	// unsafe.SliceData(buffer[:])
+	bytesRead := os.Read(fd, c.Pointer(unsafe.SliceData(buffer[:])), unsafe.Sizeof(buffer)-1)
+	if bytesRead == -1 {
+		c.Printf(c.Str("read error\n"))
+		os.Close(fd)
+		return
+	}
+
+	// Ensure that the buffer is null-terminated
+	buffer[bytesRead] = c.Char(0)
+	c.Printf(c.Str("Read %ld bytes: %s\n"), bytesRead, &buffer[0])
+
+	// Close file
+	os.Close(fd)
+}