{reply}: Add conversation-summary delta updates and likely-next preload queue

[moldovancsaba]

Objective

Deliver conversation-summary delta updates and bounded likely-next preload for the native sidebar.

Unified Context

Parent issue: #163.

The sidebar should not need broad list reloads when one conversation changes. It should update only the affected row and keep a bounded warm set for likely-next operator navigation.

Problem

One-row changes can still drag broad conversation-list work into the hot path, which wastes local compute and makes the workspace feel slower than necessary.

Goal

Keep conversation summary updates incremental and preload only a bounded set of likely-next conversations.

Scope

In Scope

• conversation-summary versioning / delta updates
• one-row update path for preview, sort position, unread, and latest message metadata
• bounded background warming for likely-next conversations
• native sidebar behavior aligned to these cheaper updates

Out of Scope

• loading full message histories for all warmed conversations
• rebuilding every conversation row after every message or profile event
• generic infinite prefetching without bounds

Execution Prompt

Implement an incremental conversation-summary lane. When one conversation changes, update only that row, reorder if required, and warm only a bounded next set of likely operator targets.

Constraints

• passive reads must still come from prepared local state
• warm caches must remain bounded and disposable
• conversation ordering logic must stay deterministic

Acceptance Checks

• a new message updates only the affected sidebar row by default
• the sidebar can reorder a changed conversation without broad list rebuilds
• bounded likely-next warming exists for adjacent or recent conversations
• non-selected rows preload summaries only, not whole-thread history by default

Dependencies

• Parent: {reply}: Native workspace hot path and incremental hydration #163
• Align with the conversation-list section of docs/NATIVE_WORKSPACE_HOT_PATH_SSOT.md

Risks

• stale ordering if versioning is weak
• overly aggressive preload can increase memory pressure without real UX benefit

Delivery Artifact

An incremental conversation-summary update path plus bounded likely-next preloading for the native sidebar.