[file.utils] add preflight_source_aware

MIGRATION.md

@@ -617,6 +617,16 @@ serialization of every cell. Set `copy_through=True` only when source-aware
 writers may copy untouched cells unchanged and intentionally skip their layer
 mapping; persistently accessed cells are mapped and no longer copied through.
 
+`preflight_source_aware(...)` preserves that per-cell provenance. It returns a
+borrowing `OverlayLibrary`, skips source-backed cells completely, and applies
+pattern sorting, named-layer validation, safe empty-cell pruning, and
+repeated-shape wrapping only to cells without reusable source provenance.
+Library name order is retained because sorting source-backed cells would
+require loading them. Always use the returned overlay for writing, and keep the
+original lazy source open until the write finishes. The comprehensive
+`preflight(...)` operation continues to materialize every cell when sorting is
+enabled.
+
 Generic borrowing views no longer expose GDS-specific `raw_struct_bytes()`,
 `can_copy_raw_struct()`, or forwarded `library_info` attributes. Keep the
 owning GDS source or the metadata returned by `readfile()` when direct access is