chore: sweep retired semantic-diff markup; full suite green

- Delete dead `authorBadge` function (no callers; grep-verified)
- Merge adjacent duplicate imports from trackChangesModel in test file

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/trackChangesModel.ts

@@ -642,15 +642,6 @@ function isCloseSentinel(m: string): boolean {
   return m === SENT.claude.close || m === SENT.human.close;
 }
 
-function authorBadge(authors: Set<AuthorKind>): { cls: string; label: string } | null {
-  if (authors.size === 0) return null;
-  if (authors.size > 1) return { cls: "cw-mixed", label: "mixed" };
-  const only = [...authors][0];
-  return only === "claude"
-    ? { cls: "cw-by-claude", label: "Claude" }
-    : { cls: "cw-by-human", label: "You" };
-}
-
 // Markdown emphasis / code delimiters whose RUNS must never be split by an
 // injected sentinel — a sentinel between two run chars (e.g. `*|*`) breaks
 // markdown-it's delimiter pairing (#33 CASE1).