Design a feature flag implementation and rollout strategy

You are a senior engineer designing a feature flag system for a new feature. Help me implement this cleanly from the start.

Language / framework: {{LANGUAGE_FRAMEWORK}}
Feature description: {{FEATURE_DESCRIPTION}}
Rollout plan (e.g., '10% of users, then 50%, then 100% over 2 weeks' or 'internal employees first'):
{{ROLLOUT_PLAN}}
Existing flag infrastructure (if any — SDK name, config system, or 'none'):
{{EXISTING_FLAG_INFRA}}

Complete these steps:
1. Recommend where to place the flag check in the codebase (entry point, middleware, service layer, component). Explain the trade-off of checking early vs. late.
2. Write the flag check code snippet for {{LANGUAGE_FRAMEWORK}}, following these rules: (a) the flag must default to OFF if the config is missing, (b) the code path for flag=false must be the existing behavior with no changes, (c) avoid nesting flag checks inside flag checks.
3. Define the flag's data structure: name (use a specific name, not 'my_flag'), type (boolean, string variant, percentage), and the context keys used for targeting (user ID, account tier, region, etc.).
4. Write a checklist for the engineer to complete before each rollout phase (smoke test criteria, metrics to watch, rollback trigger conditions).
5. Write the flag removal plan: what code to delete, what tests to update, and what the PR description should confirm when the flag is retired.
6. Flag if the described rollout plan requires session stickiness or consistent hashing to avoid users flickering between experiences, and note how to handle it.