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/embed/write-esp32/main.go

@@ -0,0 +1,31 @@
+package main
+
+import (
+	_ "unsafe"
+
+	"github.com/goplus/lib/c"
+)
+
+//go:linkname write C.write
+func write(c.Int, *c.Char, c.SizeT) int
+
+func main() {
+	buf := c.Malloc(6)
+	c.Memset(buf, 0, 6)
+	c.Strncpy((*c.Char)(buf), c.Str("abcde"), 5)
+
+	if c.Strcmp((*c.Char)(buf), c.Str("abcde")) == 0 {
+		write(1, c.Str("pass strcmp"), 11)
+	}
+
+	if byte(c.Index((*c.Char)(buf), 0)) == 'a' {
+		write(1, c.Str("pass index"), 10)
+	}
+
+	c.Memset(buf, c.Int('A'), 5)
+	if c.Strcmp((*c.Char)(buf), c.Str("AAAAA")) == 0 {
+		write(1, c.Str("pass memeset"), 11)
+	}
+
+	write(1, (*c.Char)(buf), 5)
+}