fix(runtime): tokenizeAnsi('') returns a single empty text token, matching Ink (#95)

Ink's tokenizeAnsi has no empty-string early return — '' falls through to the
no-control-chars branch and yields [{type:'text', value:''}]. vue had an extra
`if (text.length === 0) return []` guard that diverged from Ink at the tokenizer
boundary. Removed it (the production caller sanitizeAnsi short-circuits '' before
tokenizing, so no behavior changes downstream) and flipped the test.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/runtime/src/paint/ansi-tokenizer.test.ts

@@ -44,9 +44,11 @@ describe("ansi-tokenizer", () => {
     expect(tokens[0]!.type).toBe("csi");
   });
 
-  test("returns empty for empty string", () => {
+  test("empty string returns a single empty text token (Ink parity)", () => {
+    // Ink's tokenizeAnsi('') falls through to the no-control-chars branch and
+    // returns [{type:'text', value:''}] — it has no empty-string early return.
     const tokens = tokenizeAnsi("");
-    expect(tokens).toHaveLength(0);
+    expect(tokens).toEqual([{ type: "text", value: "" }]);
   });
 
   // --- Ink parity tests ---

packages/runtime/src/paint/ansi-tokenizer.ts

@@ -322,10 +322,8 @@ const malformedFromIndex = (
 };
 
 export const tokenizeAnsi = (text: string): AnsiToken[] => {
-  if (text.length === 0) {
+  // No empty-string early return: Ink falls through to the no-control-chars
-    return [];
+  // branch so tokenizeAnsi('') === [{type:'text', value:''}] (ansi-tokenizer.ts).
-  }
-
   if (!hasAnsiControlCharacters(text)) {
     return [{ type: "text", value: text }];
   }