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/targets.yml

@@ -3,7 +3,7 @@ name: Targets
 
 on:
   push:
-    branches: 
+    branches:
       - "**"
       - "!dependabot/**"
   pull_request:
@@ -43,5 +43,5 @@ jobs:
 
       - name: Build targets
         run: |
-          cd _demo/targetsbuild
+          cd _demo/embed/targetsbuild
           bash build.sh