feat(bgm): allow $variants to take multiple csv sources

Expand $variants to accept an array of csv paths, concatenating their
rows. Detect a path by a .csv suffix on the first line instead of a
newline, so inline csv and paths are self-documenting and the rule
applies uniformly to single values and array elements.

docs/bgm-format.md

@@ -37,9 +37,11 @@ bruce,[]
 
 ### Inline vs file
 
-`$variants` can be a file/URL path *or* an inline CSV string. If the value
-contains a newline it is inline CSV; otherwise it is a path. In YAML a block
-scalar (`|`) is the natural way to write inline CSV; in JSON you'd use `\n`.
+`$variants` can be a single source or an array of sources. Each source is a
+file/URL path if its first line ends in `.csv`, otherwise it is inline CSV.
+This keeps the two forms self-documenting and applies the same rule to single
+values and array elements alike. In YAML a block scalar (`|`) is the natural
+way to write inline CSV; in JSON you'd use `\n`.
 
 ```yaml
 $variants: |
@@ -47,9 +49,21 @@ $variants: |
   string,string,[number;number;number;number]
   fish,Fish,[0;0;5;2]
   grain,Grain,[1;0;5;2]
-  wood,Wood,[2;0;5;2]
 ```
 
+An array of sources concatenates their rows. This lets one part definition
+pull from several CSVs with different schemas — e.g. a deck where the regular
+cards share a face sheet but the jokers have their own:
+
+```yaml
+$variants:
+  - ./cards.csv
+  - ./jokers.csv
+```
+
+Each source is parsed with its own schema, and its rows extend the original
+object independently.
+
 ### CSV conventions
 
 CSV is parsed with `typed-csv`: