Live compaction SSE lifecycle

Chose a complete Anthropic SSE lifecycle for successful or null live-compaction summaries, but an errored stream without message_stop when summary generation rejects, over converting failures into successful empty turns because clients could then accept an empty compaction and wipe context. While pending, emit protocol-safe ping events; success/null follows message_start → content_block_start → ping* → content_block_delta → content_block_stop → message_delta → message_stop.