docs(manual): add spaced repetition terminology guide

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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+# Spaced Repetition
+
+Spaced repetition is a learning technique where cards are reviewed at increasing intervals based on how well you remember them. Kioku uses the FSRS (Free Spaced Repetition Scheduler) algorithm to schedule reviews.
+
+## Card States
+
+Each card is in one of the following states:
+
+- **New** — A card that has never been studied. It has no review history and is waiting for its first learning session.
+- **Learning** — A card currently being learned for the first time. It goes through short learning steps (e.g. 1 minute, then 10 minutes) before graduating to a review card.
+- **Review** — A card that has graduated from the learning phase. It is reviewed at increasingly longer intervals (days, weeks, months) as long as you keep remembering it.
+- **Relearning** — A review card that you forgot (lapsed). It re-enters short learning steps, similar to the learning phase, before returning to the review schedule.
+
+## Key Terminology
+
+- **Lapse** — When you fail to recall a review card. The card moves from the Review state to the Relearning state. A high lapse count may indicate that a card is a "leech" and should be reformulated.
+- **Interval** — The number of days between reviews for a card in the Review state.
+- **Ease / Stability** — A factor that determines how quickly the interval grows. In FSRS, this is represented by the stability and difficulty parameters.
+- **Graduation** — When a card in the Learning state completes all learning steps and becomes a Review card.
+- **Leech** — A card that has lapsed many times. Leeches are typically cards that are poorly written or too difficult, and should be rewritten or broken into simpler cards.
+
+## Review Ratings
+
+When reviewing a card, you rate your recall:
+
+- **Again** — You did not remember the card. If it was a Review card, this counts as a lapse.
+- **Hard** — You recalled the card but with significant difficulty.
+- **Good** — You recalled the card with acceptable effort.
+- **Easy** — You recalled the card effortlessly.
+
+## How It Works
+
+1. New cards enter the **Learning** phase with short intervals.
+2. After completing learning steps, cards **graduate** to the **Review** phase.
+3. Each successful review increases the interval. The better your rating, the larger the increase.
+4. If you forget a card (rate it **Again**), it **lapses** and enters **Relearning**.
+5. After completing relearning steps, the card returns to **Review** with a reduced interval.