Track the packaged CN data files (the data/ ignore rule matched src/simurg/cn/data)

Found by the CN cloud validation: mask_GRCh3{7,8}.bed and cytoBand_GRCh3{7,8}.txt.gz were
never committed nor shipped in the wheel. The ignore rule now covers only the top-level data/.

.gitignore

@@ -1,5 +1,5 @@
 # vendor / large data — never commit
-data/
+/data/
 refs/
 _phase0_archive/
 *.idat