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.

packages/bgm/src/variants.test.ts

@@ -48,7 +48,7 @@ describe('parseCsvData', () => {
 });
 
 describe('expandVariants', () => {
-  it('parses inline CSV when the value contains a newline', () => {
+  it('parses inline CSV when the first line does not end in .csv', () => {
     const rows = expandVariants('a,b\nstring,number\nx,1', 'pkg/def.yaml', new Map(), 'src');
     expect(rows).toEqual([{ a: 'x', b: 1 }]);
   });
@@ -61,15 +61,30 @@ describe('expandVariants', () => {
     expect(rows).toEqual([{ seat: 0 }, { seat: 1 }]);
   });
 
+  it('concatenates rows from an array of csv paths', () => {
+    const defs = new Map<string, DefFile[]>([
+      ['pkg/parts/a.csv', [defFile('pkg/parts/a.csv', 'id\nstring\nred')]],
+      ['pkg/parts/b.csv', [defFile('pkg/parts/b.csv', 'id\nstring\nblack')]],
+    ]);
+    const rows = expandVariants(['./a.csv', './b.csv'], 'pkg/parts/board.yaml', defs, 'src');
+    expect(rows).toEqual([{ id: 'red' }, { id: 'black' }]);
+  });
+
   it('throws when the referenced csv is missing', () => {
     expect(() => expandVariants('./nope.csv', 'pkg/def.yaml', new Map(), 'src')).toThrow(
       /CSV not found/,
     );
   });
 
-  it('throws when $variants is not a string', () => {
+  it('throws when $variants is not a string or string array', () => {
     expect(() => expandVariants(42, 'pkg/def.yaml', new Map(), 'src')).toThrow(
-      /must be a path or inline CSV/,
+      /must be a path or inline CSV string, or an array of them/,
+    );
+  });
+
+  it('throws on an empty $variants array', () => {
+    expect(() => expandVariants([], 'pkg/def.yaml', new Map(), 'src')).toThrow(
+      /must not be empty/,
     );
   });
 });