perf(mobile): show cached child content first and drop sibling prefetch

[iscekic]

What changed

• Child session cards show the agent name, task name, and status from task metadata only. Rendering a card never fetches the child transcript.
• Opening a child session shows cached content at once. A background refresh updates it without replacing the transcript.
• The child session the user opens gets priority hydration. Speculative prefetch of sibling child transcripts no longer runs when the parent loads.
• Empty user messages no longer paint blank bubbles in the transcript.
• The scroll-to-bottom chip appears only after the user drags away from the bottom.
• The context pill shows a fractional percent, for example 0.1%, instead of 0% for tiny usage.
• Child session open timing is measured into network, storage, and render phases. A report helper computes the before/after number for each of the five journeys and names the bottleneck.

Why

The measured bottleneck was the sibling-transcript prefetch that ran when the parent loaded. It fetched every child transcript to fill card labels.

The fix removes that prefetch, keeps child cards cheap, serves cached content first, and gives the opened child priority.

Verification

• mobile: typecheck, unit, and i18n gates passed.
• mobile-device: verifier passed.
• mobile-device: spot check clean.
• web: setup-smoke did not pass (advisory while unhardened).

Cold open — tap to first useful child content.

ux-audit-warm-reopen.mp4.trim.mp4

Warm reopen — cached child content shown at once.

case.mp4.trim.mp4

Return to parent — child reopen measured correctly.

case.mp4.trim.mp4

Many children — one child opened, siblings not prefetched.

ux-audit-cold-open.mp4.trim.mp4

Slow network — cached content served while the refresh is pending.

ux-audit-child2-cold-open.mp4.trim.mp4

Spot check — clean.

case.mp4.trim.mp4

Web profile setup-smoke test failed on Chromium.

Notes for the reviewer

This PR changes mobile files only. The web setup-smoke failure is marked advisory in the evidence.

PR stack (merge bottom to top)

• level 1: perf(mobile): show cached child content first and drop sibling prefetch (stack 1/13) #2 (kwf/deliver-the-work-described-b-ad46-l1)
• level 2: perf(mobile): show cached child content first and drop sibling prefetch (stack 2/13) #3 (kwf/deliver-the-work-described-b-ad46-l2)
• level 3: perf(mobile): show cached child content first and drop sibling prefetch (stack 3/13) #16 (kwf/deliver-the-work-described-b-ad46-l3)
• level 4: perf(mobile): show cached child content first and drop sibling prefetch (stack 4/13) #17 (kwf/deliver-the-work-described-b-ad46-l4)
• level 5: perf(mobile): show cached child content first and drop sibling prefetch (stack 5/13) #18 (kwf/deliver-the-work-described-b-ad46-l5)
• level 6: perf(mobile): show cached child content first and drop sibling prefetch (stack 6/13) #19 (kwf/deliver-the-work-described-b-ad46-l6)
• level 7: kwf/deliver-the-work-described-b-ad46-l7
• level 11: perf(mobile): show cached child content first and drop sibling prefetch (stack 11/13) #20 (kwf/deliver-the-work-described-b-ad46-l11)
• level 12: kwf/deliver-the-work-described-b-ad46-l12
• level 13: perf(mobile): show cached child content first and drop sibling prefetch #21 (kwf/deliver-the-work-described-b-ad46-l13) ← this PR

[iscekic]

bot: closing — kwf experiment artifact, superseded