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/cabisret/main.go

@@ -0,0 +1,39 @@
+package main
+
+type array9 struct {
+	x [9]float32
+}
+
+func demo1(a array9) array9 {
+	a.x[0] += 1
+	return a
+}
+
+func demo2(a array9) array9 {
+	for i := 0; i < 1024*128; i++ {
+		a = demo1(a)
+	}
+	return a
+}
+
+func testDemo() {
+	ar := array9{x: [9]float32{1, 2, 3, 4, 5, 6, 7, 8, 9}}
+	for i := 0; i < 1024*128; i++ {
+		ar = demo1(ar)
+	}
+	ar = demo2(ar)
+	println(ar.x[0], ar.x[1])
+}
+
+func testSlice() {
+	var b []byte
+	for i := 0; i < 1024*128; i++ {
+		b = append(b, byte(i))
+	}
+	_ = b
+}
+
+func main() {
+	testDemo()
+	testSlice()
+}