fix: address review findings in masks substrate

- Remove dead ternary in getPreferredEdges (always returned undefined; the
  all-false object was never reached by Object.values().some(Boolean))
- Document why drawWithFeather descriptors zero the feather field in the
  compositor (analytical renderer handles softness itself; zeroing prevents
  a second JFA feather pass)
- Remove legacyType from V30→V31 migration (not consumed anywhere; a
  down-migration can map type:"freeform" back to "custom" directly)
- Add V30→V31 test coverage: skip-guard branches, freeform passthrough,
  elements without a masks array
- Add snap tests for vertical-edge (bottom) and uniform-scale handle paths

Co-authored-by: Cursor <cursoragent@cursor.com>

1 files changed, 2 insertions, 1 deletions

diff --git a/apps/web/src/services/storage/migrations/transformers/v30-to-v31.ts b/apps/web/src/services/storage/migrations/transformers/v30-to-v31.ts
index acdf4061..512fc921 100644
--- a/apps/web/src/services/storage/migrations/transformers/v30-to-v31.ts
+++ b/apps/web/src/services/storage/migrations/transformers/v30-to-v31.ts
@@ -91,9 +91,10 @@ function migrateMask({ mask }: { mask: unknown }): unknown {
 		return mask;
 	}
 
+	// "custom" is the only freeform-path mask type; a down-migration can map
+	// type === "freeform" back to "custom" without any additional marker.
 	return {
 		...mask,
 		type: "freeform",
-		legacyType: "custom",
 	};
 }