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/concat/concat.go

@@ -0,0 +1,21 @@
+package main
+
+import (
+	"github.com/goplus/lib/c"
+)
+
+func concat(args ...string) (ret string) {
+	for _, v := range args {
+		ret += v
+	}
+	return
+}
+
+func main() {
+	result := concat("Hello", " ", "World")
+	c.Fprintf(c.Stderr, c.Str("Hi, %s\n"), c.AllocaCStr(result))
+}
+
+/* Expected output (stderr):
+Hi, Hello World
+*/