Toolchain fixes and native-hello example

[donhiatt]

Summary

Draft PR bundling local master commits not yet on upstream:

• calc: forward-declare yylex in bc.y for modern Clang
• env.sh / Makefile: derive NPROC via sysctl on macOS; wire through Docker builds
• Docker: ignore host ROOT, seed ROOT layout for mk test
• examples: add native-hello sample (C, build script, gitignore)

No additional commits were made for this PR; branch cursor/toolchain-and-native-hello matches local master at push time.