emit a source map from packed output back to blobs #12

when an agent reasons over packed context and wants to edit, it has to re-resolve paths by hand. proposal: a sidecar map from every kept span to (path, line_start, line_end, blob_hash) so edits can be applied without refetching the whole repo.

yes, and the map must survive elision: elided spans map to their stub line. otherwise the map lies, and a lying map is worse than none.