perf: prioritize child session opens

[iscekic]

What changed

• Child cards show task metadata without loading each child transcript.
• The selected child receives priority over background child work.
• Cached transcripts appear immediately while a background refresh merges new content.
• Mobile and web show a refresh indicator without replacing the transcript.
• Child-open timing separates network, storage, and render costs.
• Refreshes preserve pagination, message order, deduplication, and existing content after errors.

Why

Child cards previously loaded and inspected child transcripts before selection. This added network and storage work for sibling sessions.

The selected child now starts a user-priority fetch. Background refreshes reuse the existing transcript and pagination path.

Verification

• Driver checks: per-slice checks passed
• mobile: typecheck + unit + i18n gates passed
• mobile-device: verifier passed
• web: setup-smoke did not pass

The many-card screenshot shows correct child cards without eager transcript content.

The many-children screenshot shows the parent session with multiple child cards.

The return-flow screenshot shows the correct child card after returning to the parent.

The return-flow screenshot shows the child sheet after returning to the parent session.

The web screenshot shows the failed setup smoke result.

Notes for the reviewer

The evidence does not include before-and-after numbers for the five required journeys.

The web setup smoke check did not pass.

PR stack (merge bottom to top)

• level 1: perf: prioritize child session opens (stack 1/2) #2 (kwf/deliver-the-work-described-b-ad46-l1)
• level 2: perf: prioritize child session opens #3 (kwf/deliver-the-work-described-b-ad46-l2) ← this PR