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>

.github/workflows/test_demo.sh

@@ -1,11 +1,11 @@
 #!/bin/bash
 set -e
 
-# llgo run subdirectories under _demo and _pydemo that contain *.go files
+# llgo run subdirectories under _demo that contain *.go files
 total=0
 failed=0
 failed_cases=""
-for d in ./_demo/* ./_pydemo/*; do
+for d in ./_demo/go/* ./_demo/py/* ./_demo/c/*; do
   if [ -d "$d" ] && [ -n "$(ls "$d"/*.go 2>/dev/null)" ]; then
     total=$((total+1))
     echo "Testing $d"