RunConfig.save() crashes with callbacks that have non-serializable state

## Problem

`_RunConfig.save()` uses `dataclasses.asdict()` to serialize the run configuration to JSON. `asdict()` internally calls `copy.deepcopy()` on every field value, including the `callbacks` list. Any callback with non-copyable runtime state (e.g. `threading.Event`, `threading.Thread`, file handles, connections) will crash with a `TypeError` during save.

This affects any callback that maintains runtime state beyond simple config values — for example `SystemMetricsMonitor` which uses a background thread and a `threading.Event` for coordination.

## Expected behavior

Saving a run config should serialize only the callback's configuration, not its runtime state. The serialization path should not require `deepcopy` of live objects.

## Resolution

Merging #54 (unified serialization layer) should address this by replacing `asdict()` with `serialize()`, which uses the `__getstate__` protocol as the serialization boundary rather than recursively deepcopying internal state.

Start at `_RunConfig.save()` and inspect how `dataclasses.asdict()` handles the callbacks list. Review the unified serialization layer in #54, especially `serialize()` and the `__getstate__` boundary. Done means saving a callback with non-serializable runtime state serializes its configuration without requiring deepcopy or raising `TypeError`.

Written by the indexing model from the issue text.