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_parser_fixtures.py

@@ -74,6 +74,7 @@ def test_parse_list_timeline_fixture_with_visibility_wrapper(fixture_loader) ->
     assert cursor == "list-cursor"
     assert tweets[0].author.verified is True
     assert tweets[0].lang == "zh"
+    assert tweets[0].is_subscriber_only is True
 
 
 def test_fetch_user_list_with_fixture(monkeypatch, fixture_loader) -> None: