Fix scoped source storage paths

[bittoby]

Summary

Fixes shared source storage so approved/uploaded line and category sources are written to the same scope their manifest paths reference.

What Changed

• Added scope-aware source directory resolution.
• Own-scope sources write to the product source/ directory.
• Line-scope sources write to the product-line source/ directory.
• Category-scope sources write to the category/vendor source/ directory.
• Added tests to verify physical source paths match the manifest-relative paths.

Validation

• npm test
• npm run build

Fixes #12