feat: detect subscriber-only tweets via tweetInterstitial (#33)

Extract visibility metadata from TweetWithVisibilityResults wrapper
before unwrapping. Adds is_subscriber_only field to Tweet model,
with full serialization roundtrip and test coverage.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

tests/fixtures/list_timeline.json

@@ -15,6 +15,10 @@
                       "tweet_results": {
                         "result": {
                           "__typename": "TweetWithVisibilityResults",
+                          "tweetInterstitial": {
+                            "__typename": "TweetInterstitial",
+                            "text": { "rtl": false, "text": "Subscribe to @lister to see this post" }
+                          },
                           "tweet": {
                             "__typename": "Tweet",
                             "rest_id": "700",