refactor: importers write through write_if_changed

game_dat_importer and arcade_parent_importer each kept an exact-text
copy of write_if_changed, so a redump refresh rewrote every catalog for
its fetched date alone. Both now use artifacts.write_if_changed, which
treats fetched as a timestamp. A No-Intro rebuild still rewrites its
catalogs, since asset_updated_at moves with it, so check_freshness keeps
reading a fetched date no older than the pack.

scripts/artifacts.py

@@ -19,6 +19,7 @@ import re
 _TIMESTAMP_PATTERNS = [
     re.compile(r'"generated_at":\s*"[^"]*"'),  # database.json
     re.compile(r'"imported_at":\s*"[^"]*"'),  # provenance snapshots
+    re.compile(r'"fetched":\s*"[^"]*"'),  # games/catalogs/
     re.compile(r"\*Auto-generated on [^*]*\*"),  # README.md
     re.compile(r"\*Generated on [^*]*\*"),  # docs site pages
     # The decorated pages carry the same stamp again as a rendered element,