Add two-phase build with host asyncify transformation

- Move asyncify from Docker to host to avoid memory issues
- Auto-download Binaryen v121 (v125 has regression bug)
- Use -O1 for debug builds (V8 local count limit)
- Remove asyncify flags from linker (handled by wasm-opt)
- Document two-phase build in build.md

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.gitignore

@@ -52,3 +52,4 @@ wxwidgets-clean/
 *.log
 *.tmp
 output/
+tools/