Manage React feature flags across channels, gate tests conditionally, and debug flag-specific test failures.

• Four flag files control defaults and channel-specific overrides (canary, www, React Native, test renderer) with __VARIANT__ flags simulating gatekeepers tested in both states
• Use @gate flagName pragma to skip tests entirely when a flag is unavailable, or inline gate() to branch assertions when behavior differs
• Adding a new flag requires entries in the main file plus all fork files; set to __VARIANT__ in www/native-fb forks if the flag should vary by channel
• Common pitfalls include forgetting to test both __VARIANT__ states, using @gate for behavior differences instead of inline gate(), and omitting fork file entries