feat(bgm): add surface mounting and setup surfaces

docs/bgm-format.md

@@ -193,11 +193,29 @@ tables. It does not affect part placement; placement lives in the state store
 (see §4). A surface need not cover every part — parts with no matching route on
 this surface are simply not shown.
 
+A surface also declares how it is **mounted**: as the root table surface, on a
+HUD area, or as a child of another surface. `mount` is always an object, with
+`x`, `y`, and `rotation` (defaulting to `0`) anchoring it like a route. The
+`kind` selects the mount type:
+
+- `table` — the root table surface (default).
+- `hud` — mounted to a HUD area, e.g. a player's hand.
+- `child` — mounted relative to a parent surface. A surface lists its
+  `children` (`type#id` refs) so a surface can be repeated, like a player
+  board; each child is mounted relative to its parent's anchor.
+
 ```yaml
 type: board
 id: harbor
 role: surface
 size: [300, 200]
+mount:
+  kind: table
+  x: 0
+  y: 0
+  rotation: 0
+children:
+  - board#player
 layout:
   - route: /dock/:seat
     candidates:
@@ -208,19 +226,49 @@ layout:
     rotation: 0
 ```
 
+```yaml
+type: hud
+id: hand
+role: surface
+size: [200, 100]
+mount:
+  kind: hud
+  area: bottom-left
+```
+
+```yaml
+type: board
+id: player
+role: surface
+size: [200, 200]
+mount:
+  kind: child
+  x: 100
+  y: 50
+  rotation: 0
+```
+
 ### setup
 
-`setup` seeds the state store. Each valid game state is a valid setup.
+`setup` seeds the state store: the enabled surfaces and the part placement.
+Each valid game state is a valid setup.
 
 ```yaml
 role: setup
 type: game
 id: main
+surfaces:
+  - board#harbor
+  - hud#hand
 setup:
   /dock/0: harbor:boat#fleet
   /deck: 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.
 
 The string can either be a one part string, or a type without an id.
@@ -233,13 +281,18 @@ When id is omitted, it expands to all parts in that type during game state initi
 
 ### Game state
 
-The board's state is a **state store**: a map from path to a **stack** of
-parts. It is the authoritative record of where every part is placed.
+The board's state is a **state store**: the set of **enabled surfaces** and a
+map from path to a **stack** of parts. It is the authoritative record of which
+surfaces are in play and where every part is placed.
 
 A path is a URL path with named params, like `/dock/1`.
 
 A part is identified by a `package:type#id` string.
 
+A surface is enabled or disabled; a disabled surface is not rendered. Setup
+seeds the enabled set (see §3), and it changes at runtime as the game
+progresses (e.g. enabling the main board after an expansion-chooser scene).
+
 ### Routing
 
 A route is a **visualization route**: it maps a part to a location on a