- Make `MessageContent` own the original SDK content blocks and derive
rendering through borrowed views, removing the separately maintained
`UserMessage.chunks` vector.
- Retain assistant content-block metadata and payloads while revealing
streamed text through a source cursor, without a second pending-text
buffer or per-block shared ownership.
- Preserve existing v1 IDs, grouping, optimistic echo behavior, prompt
formatting, and Markdown/image identity. Standalone tool and compaction
output keeps its existing payload-retention contract.
- Reclaim spare source-vector capacity at turn settlement or
cancellation, with a bounded trailing range and stale-turn guards. Late
output can still append normally.

This is content-ownership preparation for ACP v2.

Release Notes:

- N/A