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/go/failed/stacktrace/main.go

@@ -0,0 +1,42 @@
+package main
+
+import (
+	"fmt"
+)
+
+type MyStruct[T any] struct {
+	value T
+}
+
+func (m *MyStruct[T]) Method() {
+	fmt.Println("In generic method")
+	genericFunc[T](m.value)
+}
+
+func genericFunc[T any](v T) {
+	fmt.Println("In generic function")
+	normalFunc()
+}
+
+func normalFunc() {
+	fmt.Println("In normal function")
+	panic("panic occurs here")
+}
+
+func main() {
+	m := &MyStruct[string]{value: "hello"}
+	m.Method()
+}
+
+//Expected:
+// In generic method
+// In generic function
+// In normal function
+// panic: panic occurs here
+
+// [0x00C6D310 github.com/goplus/llgo/internal/runtime.Rethrow+0x2f, SP = 0x60]
+// [0x00C6CF44 github.com/goplus/llgo/internal/runtime.Panic+0x2d, SP = 0x50]
+// [0x00C69420 main.normalFunc+0xf, SP = 0xa8]
+// [0x00C69564 main.genericFunc[string]+0x18, SP = 0x74]
+// [0x00C694A8 main.(*MyStruct[string]).Method+0x1f, SP = 0x84]
+// [0x00C6936C main+0x4, SP = 0x40]