[wf9a5m75]

These callbacks show that a long list is being put on the map. They mean the same as in js-sdk-react (MapConductor/js-sdk-react#17) and android-sdk-compose (MapConductor/android-sdk-compose#16):

• onAddStart(total) comes when the list brings states that are not on the map yet.
• onAddProgress(added, total) comes at most every 100 ms, and once more at added == total.
• onAddEnd(total) comes when all of them are on the map. That is the moment each state's onAdded fires; for tiled states, it is when their tiles are handed to the map.
• A state removed before it arrives no longer counts. A new list that adds states starts over, and the earlier list gets no onAddEnd. A list that only updates or removes states calls nothing.
• Callbacks run on the main actor and never during a view update.

How it works on iOS. The collections are values rebuilt on every content pass, so they keep no count themselves. Instead:

• Circles(...) and the others put an AddProgressRequest into MapViewContent, but only when a callback is set.
• MapViewBase, which every provider except the template wraps its map in, keeps the counts in a @StateObject for the life of the map view. No provider needs changing.
• Each collection is identified by where it is written: #fileID, #line and #column defaults, plus its order when the same line repeats in a loop. It starts from nothing when it leaves the content and comes back, and when the map is recreated.
• An unchanged list is recognised by its array storage, so a camera frame costs nothing; the list is walked only when it changes.
• OverlayAddedListeners (@_spi(MapConductorDriver)) passes each batch that OverlayAddedTracker.notify reports to the counts.

Also: StrategyMarkerController now calls onAdded, including for markers held until the first camera, as the web and Android versions already did.

Tests: AddProgressTests (11 cases) and StrategyMarkerClearOrderTests.testOnAddedForDirectAndHeldMarkers. The whole suite passes (557) on the iPhone 17 Pro simulator.

Checked in the sample: antenna-horizons (723,800 circles, MapLibre, simulator, Release) logged start 723800, then progress 723800/723800 and end 5.7 s later.