Core tests make a throwaway blob store with blobs::scratch

blobs, store and portable each built a BlobStore in a temp directory their own
way: pid+tag twice, a uuid once. blobs::scratch(tag) is that, with a counter, so
two tests can never share a directory even if they pick the same tag. The
desktop's and ffi's temp-dir helpers stay, one per crate: sharing them would
need a test-util feature on the core crate.

DRY pass #2, batch 2, F9 (#5372).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

core/src/local/store.rs

@@ -1810,12 +1810,6 @@ mod tests {
         assert_eq!(list_labels(&conn).expect("labels")[0].count, Some(1));
     }
 
-    fn blobs(tag: &str) -> crate::sync::blobs::BlobStore {
-        let dir = std::env::temp_dir().join(format!("ts-store-blobs-{}-{tag}", std::process::id()));
-        let _ = std::fs::remove_dir_all(&dir);
-        crate::sync::blobs::BlobStore::new(dir).expect("blobs")
-    }
-
     fn tombstones(conn: &Connection) -> Vec<(String, String)> {
         let mut stmt = conn
             .prepare("SELECT entity, id FROM pending_deletes ORDER BY entity, id")
@@ -1829,7 +1823,7 @@ mod tests {
     #[test]
     fn an_attached_file_is_kept_here_and_queued_to_upload() {
         let conn = db();
-        let blobs = blobs("attach");
+        let blobs = crate::sync::blobs::scratch("attach");
         let n = note(&conn, "with a receipt");
         conn.execute("UPDATE notes SET dirty = 0", [])
             .expect("clean");
@@ -1873,7 +1867,7 @@ mod tests {
     #[test]
     fn attaching_to_a_note_that_is_gone_writes_nothing() {
         let conn = db();
-        let blobs = blobs("gone");
+        let blobs = crate::sync::blobs::scratch("gone");
         assert!(add_attachment(&conn, &blobs, "missing", "a.txt", "text/plain", b"hi").is_err());
         let rows: i64 = conn
             .query_row("SELECT COUNT(*) FROM attachments", [], |r| r.get(0))
@@ -1886,7 +1880,7 @@ mod tests {
     #[test]
     fn only_a_file_the_server_holds_leaves_a_tombstone_when_removed() {
         let conn = db();
-        let blobs = blobs("remove");
+        let blobs = crate::sync::blobs::scratch("remove");
         let n = note(&conn, "two files");
         conn.execute(
             "INSERT INTO attachments (id, note_id, url) VALUES ('synced', ?1, '/x')",