P5a review: outlines from named destinations; pool race; fast over-limit; revalidation under the limiter; Accept pdf

- pdftext/outline: resolve named destinations through the catalog /Dests and the /Names /Dests
  tree, resolve indirect /Dest, /A and /Title, and treat reader.Null (what Dict.Get returns for
  a missing key) as absent. Before this every LaTeX and Acrobat PDF had an empty outline.
  pdftest gains NamedDests; TestOutlineNamedDestinations.
- api: pool() always goes through its Once (data race on concurrent first use);
  TestDefaultPoolConcurrentFirstUse. Revalidation of a stale cache entry counts against the
  caller/host limit.
- pdftext/runner: output over MaxText kills the child at once instead of waiting for the
  deadline; TestRunnerTooMuchTextFailsFast.
- fetch: Accept includes application/pdf;q=0.8.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/fetch/pdf.go

@@ -101,7 +101,7 @@ func (f *Fetcher) doFetch(ctx context.Context, rawURL, etag, lastModified string
 		return nil, errf(400, "", "invalid url")
 	}
 	req.Header.Set("User-Agent", f.ua())
-	req.Header.Set("Accept", "text/html,application/xhtml+xml,application/json;q=0.9,text/plain;q=0.8,*/*;q=0.1")
+	req.Header.Set("Accept", "text/html,application/xhtml+xml,application/json;q=0.9,text/plain;q=0.8,application/pdf;q=0.8,*/*;q=0.1")
 	if etag != "" {
 		req.Header.Set("If-None-Match", etag)
 	}