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/syncdebug/syncdebug.go

@@ -0,0 +1,31 @@
+package main
+
+import (
+	"io"
+	"os"
+	"sync"
+	"unsafe"
+
+	llsync "github.com/goplus/lib/c/pthread/sync"
+)
+
+type L struct {
+	mu sync.Mutex
+	s  string
+	i  int
+	w  io.Writer
+}
+
+func main() {
+	l := &L{s: "hello", i: 123, w: os.Stdout}
+	println("sizeof(L):", unsafe.Sizeof(L{}))
+	println("sizeof(sync.Mutex):", unsafe.Sizeof(sync.Mutex{}))
+	println("sizeof(llsync.Mutex):", unsafe.Sizeof(llsync.Mutex{}))
+	println("l:", l, "l.s:", l.s, "l.i:", l.i, "l.w:", l.w)
+	l.mu.Lock()
+	println("locked")
+	println("l:", l, "l.s:", l.s, "l.i:", l.i, "l.w:", l.w)
+	l.w.Write([]byte(l.s))
+	l.w.Write([]byte("\n"))
+	l.mu.Unlock()
+}