Add buildAll() to src/tofu/build.d — sequential builder with:
- Empty plan → 'nothing to build' no-op
- Recipe existence pre-check → fail-fast on missing
- Skip-if-exists: don't rebuild when output present (unless --force)
- Separator line: U+2500 box chars '──── building <name> (i/n) ────'
- Fail-fast: halt immediately on first BuildException
6 unittests: all succeed, first fails, empty plan, skip existing,
force rebuild, missing recipe.
