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/test_client.py

@@ -641,6 +641,7 @@ class TestParseTweetResult:
         tweet = parse_tweet_result(wrapped)
         assert tweet is not None
         assert tweet.id == "1234567890"
+        assert tweet.is_subscriber_only is False
 
     @patch("twitter_cli.client._get_cffi_session")
     @patch("twitter_cli.client._gen_ct_headers", return_value={})