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/cppintf/foo/bar/bar.cpp

@@ -0,0 +1,15 @@
+#include <stdio.h>
+#define interface struct
+
+interface ICallback {
+	virtual int val() = 0;
+	virtual double calc(double v) = 0;
+};
+
+extern "C" void f(ICallback* cb) {
+	printf("val: %d\ncalc(2): %lf\n", cb->val(), cb->calc(2));
+}
+
+void g(ICallback* cb) {
+	f(cb);
+}