docs: describe ordered setup placements

Document that setup is an ordered list of placements, each moving its parts to a path, in the format reference and the tabletop plan.

docs/bgm-format.md

@@ -268,19 +268,24 @@ surfaces:
   - board#harbor
   - hud#hand
 setup:
-  /dock/0: harbor:boat#fleet
-  /deck: harbor:card
+  - path: /dock/0
+    parts: harbor:boat#fleet
+  - path: /deck
+    parts: harbor:card
 ```
 
 `surfaces` lists the surfaces enabled at the start. A surface not listed is
 disabled and not rendered. When `surfaces` is omitted, all surfaces are
 enabled.
 
-The value on a setup path can be either a string, or a string list.
+`setup` is an **ordered list of placements**. Each placement moves its `parts`
+to its `path`, and entries are applied in order — so a part listed in a later
+placement ends up on that placement's path. This makes a setup read like "deal
+the deck, then move these cards to the flop".
 
-The string can either be a one part string, or a type without an id.
-
-When id is omitted, it expands to all parts in that type during game state initialization.
+`parts` can be a single part id, a bare type without an id, or a list of
+either. A bare type expands to all parts of that type during game state
+initialization.
 
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