Separate the code scan from the AI fill in the catalog flow

localize_catalog no longer re-runs generate_strings_catalog — scanning the code is its own
lane (generate_strings_catalog), so you can refresh the English catalog without ever invoking
the AI. The three stages are now independent fastlane invocations:
  generate_strings_catalog (scan) -> download_catalog_strings (download) -> localize_catalog (fold + AI-fill)
localize_catalog errors if the catalog hasn't been generated yet.

Author: jkmassel

fastlane/lanes/localization_catalog.rb

@@ -97,27 +97,26 @@
     UI.success("Downloaded GlotPress translations for #{locales.values.uniq.size} locale(s). Next: run localize_catalog (same locales:).")
   end
 
-  # LOCALIZE (steps 1 + 3) — build Localizable.xcstrings from the ALREADY-DOWNLOADED strings: scan the code into
-  # the English catalog, fold the GlotPress translations in (human => translated), and AI-fill the cells they
-  # leave empty (=> needs_review). Run `download_catalog_strings` (or `download_localized_strings`) first.
+  # LOCALIZE (fold + AI-fill) — fill the per-locale translations into the EXISTING Localizable.xcstrings: fold
+  # the downloaded GlotPress translations in (human => translated), then AI-fill the cells they leave empty
+  # (=> needs_review). It does NOT scan the code and does NOT download — so each stage is its own invocation:
+  #   generate_strings_catalog  (scan)  ->  download_catalog_strings  (download)  ->  localize_catalog  (this).
+  # Running the scan separately means you can refresh the catalog without ever touching the AI.
   #
   # Uploading the AI drafts back to GlotPress as needs-review (the eventual "step 4") is a separate step, not
   # done here — it builds on the existing GlotPress import integration (cf. gp_update_metadata_source).
   #
   # STAGED, NOT SHIPPED: Localizable.xcstrings isn't the runtime store yet (the app still ships
   # Localizable.strings), so this only pre-populates it for the cutover — it changes nothing users see.
   #
-  # MANUAL ONLY — not wired into download_localized_strings or any CI step: it runs xcstringstool extraction,
-  # calls the translation API (cost), and commits a large catalog. Set ANTHROPIC_API_KEY for the AI rung; scope
-  # a cheap run with `locales:fr`.
-  desc 'Localize step: build Localizable.xcstrings from downloaded strings — scan + fold + AI-fill (staged; manual, not CI)'
+  # MANUAL ONLY — not wired into download_localized_strings or any CI step: it calls the translation API (cost)
+  # and commits a large catalog. Set ANTHROPIC_API_KEY for the AI rung; scope a cheap run with `locales:fr`.
+  desc 'Localize step: fold downloaded GlotPress translations + AI-fill into the existing Localizable.xcstrings (run generate_strings_catalog first)'
   lane :localize_catalog do |options|
+    UI.user_error!("#{LOCALIZABLE_CATALOG} not found — run generate_strings_catalog first") unless File.exist?(LOCALIZABLE_CATALOG)
     locales = catalog_target_locales(options[:locales])
 
-    # 1. Scan the code into the English catalog (create / update + reconcile changed sources).
-    generate_strings_catalog
-
-    # Fold the downloaded human translations in (=> translated), then (3) AI-fill the cells they leave empty.
+    # Fold the downloaded human translations in (=> translated), then AI-fill the cells they leave empty.
     catalog = JSON.parse(File.read(LOCALIZABLE_CATALOG))
     written = CatalogStrings.fold_translations!(
       catalog,