Move materialization write ids to changes

[marcus-pousette]

Summary

• move local materialization write ids from event.writeIds to each affected change's source.writeIds
• keep operation source metadata on the same per-change source object
• strip per-change write ids before broadcasting materialization events to peer browser clients
• update conformance and sync write-id tests

Example

Before:

{
  headSeq: 42,
  writeIds: ['local-write-1'],
  changes: [
    { kind: 'insert', node: 'a', source: { operation } },
    { kind: 'payload', node: 'b', source: { operation } },
  ],
}

After:

{
  headSeq: 42,
  changes: [
    { kind: 'insert', node: 'a', source: { operation, writeIds: ['local-write-1'] } },
    { kind: 'payload', node: 'b', source: { operation, writeIds: ['local-write-1'] } },
  ],
}