iOS runner: isolate collapsed-tab occupancy test from prerequisite timeouts #3800

Purpose

Make the collapsed-tab occupancy test exercise its intended exhausted tree tier on slow CI hosts.

testPlanMovesPastATreeTierWhoseCollapsedTabQueryCannotStart expects reason budget. It sometimes receives capture-failed at RunnerTests+SnapshotCapturePlanOccupancyTests.swift:529.

Evidence

The raw job logs show the same assertion on three unrelated heads:

• fix(daemon): replace fixed 15s startup kill with progress-aware startup budget #3791, 0295ee5f: job 117698966135.
• refactor(host): read every configuration value from an explicit host (#3794) #3798, 7c105337: job 117668259465.
• build(package): stop shipping Swift test sources #3785, 449b4d48: job 117419145731.

The test file is identical across these heads. #3791 changes no Swift file. Its new daemon timing step runs after the XCTest suite.

The #3791 log records a one-second snapshot_viewport timeout before private AX recovery. The other failures record a tree-processing timeout. These paths produce capture-failed before the intended budget assertion. Host timing is a suspected trigger; it is not a proven complete cause.

A focused local loop passed 3/3 times on a separate iOS 26.2 simulator. No local red reproduction or fix is proven.

Required behavior

• Isolate prerequisite viewport, orientation, keyboard, and processing work from the deliberately delayed tree operation.
• Preserve the assertion that the delayed operation was entered.
• Preserve the expected budget reason and the refusal to accept a truncated tree as healthy.
• Use a deterministic fixture or injected facts. A warm-up read alone must not claim to prevent a later timeout.
• Do not widen timeouts, add retries, skip the test, or accept both reason codes.

Completion

Show a focused reproduction that detects the unintended prerequisite timeout. Then show the corrected fixture passes and still detects the intended regression. Run the owning iOS lane.

Owner: the iOS runner test lane. Related: #2847 and #2957. Keep this fix separate from #3791's daemon startup contract.