Bound ChatModel growth in a long session

[singular0]

On-disk history is capped at History::MaxPerChannel (500) per channel, but the live ChatModel
grows without bound as messages arrive, so a session left running for days on a CM4 keeps every row
in memory.

Tasks:

• Apply the same retention limit to the live model, or deliberately virtualize older rows.
• Keep the bottom anchor and the unseen divider correct when rows are dropped from the top.

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Related to #7, which reads the same rows.