fix: make missing source trees non-fatal in preflight validator

Missing source trees (e.g. plasma-desktop with recipe.toml but not yet
extracted source/) should not block the build. Sources are fetched/extracted
by the cookbook during the build phase. The validator now warns about
missing sources but returns exit code 0.

local/scripts/validate-source-trees.py

@@ -118,7 +118,7 @@ def main():
             if recipe_path not in seen:
                 print(recipe_path)
                 seen.add(recipe_path)
-        return 1 if missing_entries else 0
+        return 0  # Missing sources are non-fatal; they get fetched during build
 
     print(f"=== Validating source trees for config: {args.config} ===")
     for package_name, recipe_dir in missing_entries: