chat: preserve interrupted human updates consistently across stop and resume #1620

An explicitly human-directed [update] can be visible while streaming, disappear when stopped, then return as a completed answer after reopening the same journal.

The live path marks the entry cut and folds it (TestExplicitUpdateStreamsAsAddressedButNotCompletedAcrossLenses currently encodes that behavior). keepPartial records an ordinary tool-free assistant message, and replay classifies that message as Answer=true. Audience and completion need independent durable representation.

Expected behavior:

• Preserve explicitly addressed partial updates as readable, interrupted human messages, both live and after resume.
• Keep unfinished operational narration collapsed; retain full details behind disclosure.
• Keep completed earlier updates and human steering exactly once through stop, retry, later work, and replay.
• Use the shared presentation path for ordinary chat, managers, task pages and nested transcripts, including narrow widths.

Found during the transition audit for #1607. Related to #1617, but distinct from generated interruption notes leaking into prose. Duplicate search found no existing issue or PR for this audience/completion mismatch.

Validation must include actual engine stop → journal → replay and shared-renderer transition tests, rather than independent snapshots that disagree about completion.