fix(ios): preserve top position through initial loading

[Tlaster]

Weibo detail could jump away from the top when comments or reposts finished loading because repeated initial-loading updates saved the placeholder-era tabs as a reload bookmark. Discover could also hide newly loaded user sections when an accessory measurement left an item bookmark that had actually been captured at the top.

Only capture reload bookmarks when entering loading from a non-loading snapshot. Record whether a layout bookmark originated at the top and carry that information into geometry changes, allowing late top sections to stay visible while preserving genuinely scrolled bookmarks through temporary offset clamping.

Validation on iPhone 17 Pro / iOS 26.5:

• The two new controller tests and the strengthened layout-to-geometry test failed on master and pass with this change.
• 28 controller regression tests, 39 native collection-view tests, and 2 diagnostic probes covering 14 initial-loading scenarios passed.
• Tests ran in an isolated simulator host with current controller/collection sources and cached Kotlin/rendering modules. The list-renderer integration test was excluded because the cached module predates that API. Full current-app build and physical-device verification were not run.

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