**Step 6 Complete**: Extract using imports from .hako source in pipeline
Changes:
- Modified collect_using_and_strip() to return (cleaned, prelude_paths, imports)
- Build imports HashMap from seen_aliases (alias -> alias mapping)
- Updated all 6 call sites to handle new 3-tuple return type
- This provides MirBuilder with info about which names are valid static box references
Next: Wire imports through extern_provider to MirBuilder (Step 7)
Related: #phase-21.8 MatI64/IntArrayCore integration
1f961b3e75