Fix: Ensure mapping is sorted in NewHeader for binary search correctness

The getMapping function uses sort.Search which requires mappings to be
sorted by Offset. While all current callers provide sorted mappings,
NewHeader is a public API that didn't enforce this invariant. This could
lead to silent data corruption if future callers or deserialized data
provide unsorted mappings. Added sort.Slice to guarantee correctness.

Author: cursoragent

packages/shared/pkg/storage/header/header.go

@@ -32,6 +32,10 @@ func NewHeader(metadata *Metadata, mapping []*BuildMap) (*Header, error) {
 		}}
 	}
 
+	sort.Slice(mapping, func(i, j int) bool {
+		return mapping[i].Offset < mapping[j].Offset
+	})
+
 	return &Header{
 		Metadata: metadata,
 		Mapping:  mapping,