Fix misleading comments about count-based progress

Update comments to accurately describe the progress tracking:
- Test uses size-weighted progress, not count-based
- Without manifest, layer-reported sizes are used (not count-based)

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Co-Authored-By: Claude <[email protected]>

Author: agners

tests/api/test_supervisor.py

@@ -359,7 +359,7 @@ async def test_api_progress_updates_supervisor_update(
         and evt.args[0]["data"]["event"] == WSEvent.JOB
         and evt.args[0]["data"]["data"]["name"] == "supervisor_update"
     ]
-    # Count-based progress: 2 layers need pulling (each worth 50%)
+    # Size-weighted progress: 2 layers need pulling
     # Layers that already exist are excluded from progress calculation
     assert events[:4] == [
         {