Harden the DDG collector with token-bucket pacing and prefer the ddgs package

TODO.md

@@ -44,7 +44,8 @@ licence-free zone only.
 - [x] Add a DuckDuckGo image-search fetcher: `tools/refcollect.py search <query> --slug <block>
       --limit N` downloads top results into `tmp/research/<block>/` with a source-URL manifest
       (no licence filter, marked `local_only`/`unchecked-local`), and `sheet` composes a local
-      contact sheet. Retries DDG's HTTP 202 rate limit with backoff. Tests in
+      contact sheet. Requests go through a token bucket plus polite jittered backoff that honours
+      `Retry-After`, and send a full Firefox header set. Tests in
       `tools/test_refcollect.py`. **Run note:** DDG rate-limits aggressively; space batches and
       run the per-venue collection below when the limit clears.
 - [ ] Collect Hollywood block reference: El Capitan facade (day/night/close-up), the El Capitan