feat(bgm): derive definition names from role.type

A definition is identified by role.type and named role.type.lang
(package.lang for packages). Code blocks declare it via role= on the
info string; real files by their filename. Blocks and files without a
matching role.type are ignored. file= still overrides the auto-name.

Relative asset and $variants paths now resolve against the markdown
file's directory (baseDir) rather than the virtual name. Same role.type
with different ids groups under one name; duplicate type#id still
errors. Migrate fixtures and example games to role=.

games/carcassonne/carcassonne.md

@@ -4,8 +4,7 @@ The base game's 24 landscape tiles, laid out on a table with a draw pile and a
 grid of placed tiles. Each tile is a single `110×110` image (A–X), sized to a
 standard `45×45` mm square.
 
-```yaml file=carcassonne.yaml
-role: package
+```yaml role=package
 id: carcassonne
 title: Carcassonne
 designer: Klaus-Jürgen Wrede
@@ -21,9 +20,7 @@ One `tile` part per distinct tile type (A–X). All tiles share the same square
 face image and a uniform size; the `$variants` CSV expands them into the 24
 tile parts.
 
-```yaml file=tiles.yaml
-role: part
-type: tile
+```yaml role=part.tile
 face: ./20AE_Base_Game_C2_Tile_A.png
 size: [45, 45, 3]
 fillet: 1
@@ -65,10 +62,8 @@ A table with a draw pile on the left and an `11×11` grid of placed tiles in the
 middle. The grid routes each tile to its `col,row` cell, spaced `45` mm apart
 so tiles sit edge to edge.
 
-```yaml file=board.yaml
-type: board
+```yaml role=surface.board
 id: board
-role: surface
 size: [600, 600]
 layout:
   - route: /draw
@@ -215,9 +210,7 @@ string,string,number,number,number
 Start with the full tile supply on the draw pile (`carcassonne:tile` expands to
 every tile of that type), then seed the grid with a few opening tiles.
 
-```yaml file=main.yaml
-role: setup
-type: game
+```yaml role=setup.game
 id: main
 surfaces:
   - board#board