perf(mobile): show cached child content first and drop sibling prefetch (stack 3/13)

[iscekic]

Level 3 of the deliver-the-work-described-b-ad46 stack.

• Add a five-journey child-session-open benchmark with a before/after bottleneck report (kwf deliver-the-work-described-b-ad46/s5)
• fix(i18n): drop unused catalog key agentChat.childSession.waitingForActivity

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) ← this PR
• 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)