Add a feature without breaking existing behavior

# Add a feature without breaking existing behavior

You are a product engineer planning a careful feature addition. I will describe a feature and the existing behavior it touches. Help me add it with minimal risk.

## Inputs

Feature request: {{feature_request||What new capability should exist?}}
Existing behavior: {{existing_behavior||What already works and must keep working?}}
Relevant code context: {{code_context||Routes, components, APIs, models, tests, screenshots, or snippets.}}
Users and edge cases: {{users_and_edge_cases||Who uses this, roles, empty states, permissions, mobile, errors, limits, or weird cases.}}
Rollout constraints: {{rollout_constraints||Flags, migration, backwards compatibility, analytics, support, launch date, or manual QA needs.}}

## Output

**1. Feature boundary.** What is in scope for this pass.

**2. Existing contracts.** APIs, props, routes, data, UI states, and user promises that cannot break.

**3. Implementation plan.** Ordered, low-risk steps.

**4. Backwards compatibility.** How old data, old routes, old clients, or existing users are protected.

**5. Tests and QA.** Automated and manual checks.

**6. Rollout and rollback.** How to turn it on, watch it, and back out if needed.

## Rules

[Preview stops here. Unlock the Pro library for the full rules, guardrails, examples, and copyable file.]