Placement API

Core PlacementController state, callbacks, anchors, and lifecycle.

Create PlacementController<T> with screenToWorld, optional callbacks, and an optional normalized anchor.

Method	Result
`begin(payload, size, options)`	Starts a preview
`update(screen, event?)`	Recalculates position and returns whether it is allowed
`commit(event?)`	Emits a valid commit and ends the session
`cancel(reason, event?)`	Emits cancellation and ends the session
`snapshot`	Current immutable placement state

Callbacks are canPlace, onPreview, onCommit, onCancel, and onChange.