audit: "3Sum" (#15) to Public Release

[Rowrow620]

First Timers Only

This issue is reserved for developers who are new to AlgoBuddy or open-source contributing.

Description of the Issue

The visualizer for 3Sum (LeetCode #15) is currently in Developer Mode ([EXP]). We need to audit its step generator, verify active line highlighting, add an automated unit test, and promote it to Public Release status (AuditStatus::Audited).

Implementation Steps

1. Run cargo run -- --dev and select 3Sum (Add category completion ratios to NeetCode Mastery Dashboard #15).
2. Verify that two-pointer movements (i, left, right) and triplet accumulation match execution steps.
3. Open src/app.rs under #[cfg(test)] mod tests and add a unit test asserting step output for [-1,0,1,2,-1,-4].
4. Open src/model/problem.rs and update audit_status(&self) to include Problem::ThreeSum.
5. Run cargo test to verify all tests pass.

Acceptance Criteria

• cargo fmt --all -- --check completes without warnings.
• cargo test passes 100% of unit tests.
• Branch targets dev.

Step-by-Step Contribution Guide

1. Claim this issue: Comment below that you are working on this issue to receive assignment.
2. Fork and Branch: Create a new branch off dev named audit-[problem-slug].
3. Make and Test Changes: Run cargo fmt --all and cargo test locally to verify build health.
4. Submit PR: Push your branch to GitHub and create a Pull Request targeting the dev branch.

Additional Information

• Contributing Guide
• README