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/py/matrix/matrix.go

@@ -0,0 +1,22 @@
+package main
+
+import (
+	"github.com/goplus/lib/py"
+	"github.com/goplus/lib/py/numpy"
+	"github.com/goplus/lib/py/std"
+)
+
+func main() {
+	a := py.List(
+		py.List(1.0, 2.0, 3.0),
+		py.List(4.0, 5.0, 6.0),
+		py.List(7.0, 8.0, 9.0),
+	)
+	b := py.List(
+		py.List(9.0, 8.0, 7.0),
+		py.List(6.0, 5.0, 4.0),
+		py.List(3.0, 2.0, 1.0),
+	)
+	x := numpy.Add(a, b)
+	std.Print(py.Str("a+b ="), x)
+}