fix(fsrs): achieve state preservation parity between Forget and Rollback

[cantalupo555]

Summary

This PR achieves state preservation parity between the Forget and Rollback operations in the FSRS (Free Spaced Repetition Scheduler). The Forget function now captures the card's full pre-forget state in the review log, and the Rollback function now restores RemainingSteps from the log, ensuring both operations consistently preserve and restore all relevant state information.

Bug Description

• What was happening: The Forget function was not capturing the card's pre-forget state (State, Due, Stability, Difficulty, ScheduledDays, RemainingSteps) in the ReviewLog — instead it recorded only the reset state. The Rollback function restored most fields from the log but omitted RemainingSteps.
• Expected behavior: Both Forget and Rollback should preserve and restore the complete card state to maintain operational parity.
• Root cause: Forget recorded minimal information in the ReviewLog rather than the card's pre-operation state. Rollback was missing the RemainingSteps restoration logic.

Changes Made

• forget.go: Capture pre-forget State, Due, Stability, Difficulty, ScheduledDays, and RemainingSteps in the ReviewLog; preserve card LastReview.
• rollback.go: Add result.RemainingSteps = log.RemainingSteps to restore RemainingSteps from the ReviewLog during rollback.
• fsrs_test.go: Update existing tests and add new test cases covering pre-state capture, ScheduledDays computation, LastReview preservation, and learning-state rollback.

Testing

• Tested locally
• Unit tests added/updated
• No regressions

Related Issues

Fixes #65

Breaking Changes

None — These are bug fixes correcting incorrect behavior. The public API signatures remain unchanged.

[ishiko732]

LGTM

[cantalupo555]

LGTM

Thank you!!!