test(bench): cover the RenderBenchPage delegator; drop an unreachable branch

codecov/patch flagged benchpage.go at 90.24%. Two fixes bring it to 100%:

- The happy-path test now calls the package-level RenderBenchPage (the
  public API) rather than the method, so the delegator's success path is
  covered within this package's own coverage profile (the CLI test that
  exercised it lives in a different package).
- githubURLForRelativeTarget's `if rel == ""` guard was unreachable: the
  leading-'#' check above it means any '#' found sits at index >= 1, so
  rel is never empty after the split. Removed it (a defensive branch must
  be drivable red/green) and noted the invariant in a comment.

Author: claude

internal/release/benchpage.go

@@ -83,9 +83,8 @@ func githubURLForRelativeTarget(target, srcDirRel string) (string, bool) {
 	if i := strings.IndexByte(target, '#'); i >= 0 {
 		rel, frag = target[:i], target[i:]
 	}
-	if rel == "" {
-		return "", false
-	}
+	// rel is always non-empty here: the guard above rejects a
+	// leading '#', so any '#' found sits at index >= 1.
 	resolved := path.Join(srcDirRel, rel)
 	if strings.HasSuffix(rel, "/") {
 		resolved += "/"

internal/release/benchpage_test.go

@@ -99,8 +99,10 @@ func TestRenderBenchPageHappyPath(t *testing.T) {
 	require.NoError(t, os.WriteFile(readme,
 		[]byte("# Bench\n\nReproduce with [`run.sh`](run.sh).\n"), 0o644))
 
+	// Exercise the package-level delegator (the public API) so its
+	// success path is covered within this package's profile.
 	out := filepath.Join(root, "pages", "benchmark.md")
-	require.NoError(t, New().RenderBenchPage(root, out))
+	require.NoError(t, RenderBenchPage(root, out))
 
 	got, err := os.ReadFile(out)
 	require.NoError(t, err)