FOrreryTerrainFeaturePlacement

The bases a feature's bake reads reach past any cover sized to its footprint: its blocks of up to 4 edit tiles and their apron (the longest daylight plus 16 m). A commit the terrain finds Pending requests them, but only its own transaction pins them, so the streamer drops the loads as stale once the refused commit returns, and an editor with no camera over them (a headless one, or a viewport looking elsewhere) never keeps them. So while Check answers Pending, a transaction that staged the feature is held: it is never committed, but it pins and requests every base the commit needs for as long as the placement waits. Each Step checks again and commits once Check answers Ok. A refusal while every base is resident (cold sculpt pages being read back) is retried for kColdRetry_s; any other refusal is final, and so is a wait past kGroundWait_s (the last try commits or refuses). A terrain re-created or removed while it waits refuses it.

A removal (BeginRemove) waits the same way for the bases under the removed feature's footprint (CheckRemove), and GetFeatureId then names the removed feature.

The commit takes the editor follow (BeginEdit just before it, FinishEdit just after), so TakeChange returns the one record that undoes the feature and the instances that followed it. The caller appends it as one editor transaction.