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/api/api.go

@@ -314,7 +314,7 @@ type v1Req struct {
 	JQ       string `json:"jq"`
 	Recover  bool   `json:"recover"`
 
-	// Pages narrows a PDF digest to "2-3"; Grep matches lines in it. Optional; task-07.
+	// Pages ("41-60") and Grep (an RE2 pattern) narrow a PDF answer in clean mode; see pdfdoc.
 	Pages string `json:"pages"`
 	Grep  string `json:"grep"`
 }