## What changed
- Load an optional package-supplied config file as the lowest-precedence
configuration layer, so every existing layer can override its values.
- Report the layer as `packagedDefaults`, including its source path, through
config diagnostics and the app-server protocol.
- Return an error when a configured packaged defaults file is missing.
## Testing
- Cover precedence across packaged, system, user, and session layers.
- Cover the missing-file error path.
GitOrigin-RevId: 52406890f4d580eedabc90bf2a9ee3affa36fee1
jif
34ecac1f2b