chore: bump transformers lower bound to >=5.12.0

Was >=4.57.6; installed version is 5.12.0. Prevents users from
accidentally resolving the old 4.x series.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

pyproject.toml

@@ -29,7 +29,7 @@ dependencies = [
     "rich>=14.2.0",
     "torch>=2.12.0",
     "torchvision>=0.27.0",
-    "transformers>=4.57.6",
+    "transformers>=5.12.0",
     "ultralytics>=8.4.66",
 ]